First, adrenalin reduces hand steadiness and increases anxiety (Basowitz, Korchin, Oken, Goldstein, & Gussack, 1956), so other causes of anxiety seem likely to reduce steadiness by promoting the release of adrenalin. Next, here's an odd reference for a more voluntary psychopharmacological cause of hand-arm unsteadiness: drug abuse (including alcohol; Kaur, Sandhu, & Sandhu, 2007). Caffeine and nicotine both impair hand steadiness, apparently without interacting (Smith, Tong, & Leigh, 1977; Heatherley, Hayward, Seers, & Rogers, 2005; Rogers et al., 2005; Richardson, Rogers, Elliman, & O'Dell, 1995; Bovim, Næss, Helle, & Sand, 1995; see also for a review of caffeine effects and original evidence of a lack of interactions with alcohol Franks, Hagedorn, Hensley, Hensley, & Starmer, 1975). Daily consumption of caffeine does not appear related though, and the effect of active caffeine can be blocked medically (Arnold, Springer, Engel, & Helveston, 1993). A small dose of propranolol may also reduce hand tremors regardless of caffeine use (Humayun, Rader, Pieramici, Awh, & de Juan, 1997), though interested readers should consider precautions and contraindications regarding propranolol, including hypoglycaemia or diabetes, asthma, and abnormal blood pressure, among several others (Rossi, 2006).
Physical exertion (Mürbe et al., 2001; Simon & Dare, 1965; Halliday & Redfearn, 1956), skill in manual labor, and sex (Gray, Sustare, & Thompson, 1953) appear relevant as well. Gray and colleagues found greater hand steadiness in men, but a more recent study found the opposite, except in women during premenstrual phases and women using oral contraceptives (Hudgens, Fatkin, Billingsley, & Mazurczak, 1988). A loosely related sex difference emerged in a study of marksmanship, suggesting men's accuracy endures longer than women's (Johnson & Merullo, 1996); incidentally, this study found no effect of caffeine on marksmanship. Also, another study found lesser effects of caffeine on motor performance in women who use caffeine regularly (Jacobson & Thurman-Lacey, 1992). A particularly old study seems to have found a weakly negative (and quite possibly spurious) correlation between performance on a mental multiplication test and hand steadiness, as well as a positive experimental effect of humidity (Stecher, 1916).
An early electroencephalographic (EEG) study emphasized the distinction between low-frequency, high-amplitude tremors, and the opposite kind, suggesting that the latter is less affected by cortical activity (Lindqvist, 1941). Two relatively recent studies produced apparently conflicting evidence about the relationship between hand and finger tremors and EEG activity: one succeeded (Isokawa & Komisaruk, 1983) and another failed to influence hand tremors with rhythmic flashes of light, even though this induced changes in EEG activity (Lakie & Combes, 1999). Other articles on hand steadiness research have been published in psychological journals to which I don't have access; the first of these even considers personality factors such as extraversion (Treadwell, 1960; Fleishman, 1956; Lovell, 1941).
Last (for this edition of this answer), some evidence supports the effectiveness of yoga for hand steadiness (Telles, Hanumanthaiah, Nagarathna, & Nagendra, 1994), finger flexibility, grip strength and endurance (Garfinkel, Schumacher, Husain, Levy, & Reshetar, 1994; Madanmohan, Jatiya, Udupa, & Bhavanani, 2003; Dash & Telles, 2001; Madanmohan et al., 1992; Raghuraj, Nagarathna, Nagendra, & Telles, 1997; Raghuraj & Telles, 1997), and stability of knee extensors (Bukowski, Conway, Glentz, Kurland, & Galantino, 2006). These references were cited in a recent review that deserves mention itself (Donahoe-Fillmore, Brahler, Fisher, & Beasley, 2010).
- Arnold, R. W., Springer, D. T., Engel, W. K., & Helveston, E. M. (1993). The effect of wrist rest, caffeine, and oral timolol on the hand steadiness of ophthalmologists. Annals of Ophthalmology, 25(7), 250–253.
- Basowitz, H., Korchin, S. J., Oken, D., Goldstein, M. S., & Gussack, H. (1956). Anxiety and performance changes with minimal dose of epinephrine. AMA Archives of Neurology & Psychiatry, 76(1), 98–105.
- Bovim, G., Næss, P., Helle, J., & Sand, T. (1995). Caffeine influence on the motor steadiness battery in neuropsychological tests. Journal of Clinical and Experimental Neuropsychology, 17(3), 472–476.
- Bukowski, E. L., Conway, A., Glentz, L. A., Kurland, K., & Galantino, M. L. (2006). The effect of iyengar yoga and strengthening exercises for people living with osteoarthritis of the knee: A case series. International Quarterly of Community Health Education, 26(3), 287–305.
- Dash, M., & Telles, S. (2001). Improvement in hand grip strength in normal volunteers and rheumatoid arthritis patients following yoga training. Indian Journal of Physiology and Pharmacology, 45(3), 355–360. Retrieved from http://yoga4arthritis.com/wp-content/uploads/2013/09/20010101_ihgsnvrapfyt.pdf.
- Donahoe-Fillmore, B., Brahler, C. J., Fisher, M. I., & Beasley, K. (2010). The effect of yoga postures on balance, flexibility, and strength in healthy high school females. Journal of Women’s Health Physical Therapy, 34(1), 10–17. Retrieved from http://www.researchgate.net/publication/232207909_The_Effect_of_Yoga_Postures_on_Balance_Flexibility_and_Strength_in_Healthy_High_School_Females/file/72e7e515448e1c52be.pdf.
- Franks, H. M., Hagedorn, H., Hensley, V. R., Hensley, W. J., & Starmer, G. A. (1975). The effect of caffeine on human performance, alone and in combination with ethanol. Psychopharmacologia, 45(2), 177–181.
- Fleishman, E. A. (1956). Psychomotor selection tests: Research and application in the United States Air Force. Personnel Psychology, 9(4), 449–467.
- Garfinkel, M. S., Schumacher Jr., H. R., Husain, A. B. I. D., Levy, M., & Reshetar, R. A. (1994). Evaluation of a yoga based regimen for treatment of osteoarthritis of the hands. The Journal of Rheumatology, 21(12), 2341–2343.
- Gray, J. S., Sustare, G., & Thompson, A. (1953). An apparatus for measuring operational hand steadiness. Journal of Applied Psychology, 37(1), 57–58.
- Halliday, A. M., & Redfearn, J. W. T. (1956). An analysis of the frequencies of finger tremor in healthy subjects. The Journal of Physiology, 134(3), 600–611. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1359164/pdf/jphysiol01371-0105.pdf.
- Heatherley, S. V., Hayward, R. C., Seers, H. E., & Rogers, P. J. (2005). Cognitive and psychomotor performance, mood, and pressor effects of caffeine after 4, 6 and 8 h caffeine abstinence. Psychopharmacology, 178(4), 461–470.
- Hudgens, G. A., Fatkin, L. T., Billingsley, P. A., & Mazurczak, J. (1988). Hand steadiness: effects of sex, menstrual phase, oral contraceptives, practice, and handgun weight. Human Factors: The Journal of the Human Factors and Ergonomics Society, 30(1), 51–60.
- Humayun, M. U., Rader, R. S., Pieramici, D. J., Awh, C. C., & de Juan, E. (1997). Quantitative measurement of the effects of caffeine and propranolol on surgeon hand tremor. Archives of Ophthalmology, 115(3), 371–374.
- Isokawa, M., & Komisaruk, B. R. (1983). Convergence of finger tremor and EEG rhythm at the alpha frequency induced by rhythmical photic stimulation. Electroencephalography and Clinical Neurophysiology, 55(5), 580–585.
- Jacobson, B. H., & Thurman-Lacey, S. R. (1992). Effect of caffeine on motor performance by caffeine-naïve and familiar subjects. Perceptual and Motor Skills, 74(1), 151–157.
- Johnson, R. F., & Merullo, D. J. (1996). Effects of caffeine and gender on vigilance and marksmanship. Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 40(23), 1217–1221.
- Kaur, G., Sandhu, S. S., & Sandhu, J. S. (2007). Comparison of arm-hand steadiness for shooting perfection in armed forces and Punjab police. Anthropologist, 9(4), 299–304. Retrieved from http://www.krepublishers.com/02-Journals/T-Anth/Anth-09-0-000-000-2007-Web/Anth-09-4-000-07-Abst-PDF/Anth-09-4-299-07-393-Kaur-Gaganpreet/Anth-09-4-299-07-393-Kaur-G-Tt.pdf.
- Lakie, M., & Combes, N. (1999). The phase of postural hand tremor is not influenced by repetitive photic brain stimulation. Clinical Neurophysiology, 110(12), 2020–2025.
- Lindqvist, T. (1941). Finger tremor and the α‐waves of the electro‐encephalogram. Acta Medica Scandinavica, 108(6), 580–585.
- Lovell, C. (1941). A study of personal variation in hand-arm steadiness. The American Journal of Psychology, 54(2), 230–236.
- Madanmohan, Jatiya, L., Udupa, K., & Bhavanani, A. B. (2003). Effect of yoga training on handgrip, respiratory pressures and pulmonary function. Indian Journal of Physiology and Pharmacology, 47(4), 387–392. Retrieved from http://icyer.com/documents/18.pdf.
- Madanmohan, Thombre, D. P., Bharathi, B., Nambinarayan, T. K., Thakur, S., Krishnamurthy, N., & Chandrabose, A. (1992). Effect of yoga training on reaction time, respiratory endurance and muscle strength. Indian Journal of Physiology and Pharmacology, 36, 229–233. Retrieved from http://www.ijpp.com/IJPP%20archives/1992_36_4/229-233.pdf.
- Mürbe, D., Hüttenbrink, K. B., Zahnert, T., Vogel, U., Tassabehji, M., Kuhlisch, E., & Hofmann, G. (2001). Tremor in otosurgery: Influence of physical strain on hand steadiness. Otology & Neurotology, 22(5), 672–677.
- Raghuraj, P., Nagarathna, R., Nagendra, H. R., & Telles, S. (1997). Pranayama increases grip strength without lateralized effects. Indian Journal of Physiology and Pharmacology, 41(2), 129–133. Retrieved from http://ow.ly/tL6NG.
- Raghuraj, P., & Telles, S. (1997). Muscle power, dexterity skill and visual perception in community home girls trained yoga or sports and in regular school girls. Indian Journal of Physiology and Pharmacology, 41(4), 409–415. Retrieved from http://www.libraryofyoga.com:8080/bitstream/handle/123456789/57/II.1997.25.doc?sequence=1.
- Richardson, N. J., Rogers, P. J., Elliman, N. A., & O'Dell, R. J. (1995). Mood and performance effects of caffeine in relation to acute and chronic caffeine deprivation. Pharmacology Biochemistry and Behavior, 52(2), 313–320.
- Rogers, P. J., Heatherley, S. V., Hayward, R. C., Seers, H. E., Hill, J., & Kane, M. (2005). Effects of caffeine and caffeine withdrawal on mood and cognitive performance degraded by sleep restriction. Psychopharmacology, 179(4), 742–752.
- Rossi, S. (Ed., 2006). Australian Medicines Handbook. Adelaide.
- Simon, J. R., & Dare, C. E. (1965). Effects of physical exercise on hand steadiness. The Laryngoscope, 75(11), 1737–1740.
- Smith, D. L., Tong, J. E., & Leigh, G. (1977). Combined effects of tobacco and caffeine on the components of choice reaction-time, heart rate, and hand steadiness. Perceptual and Motor Skills, 45(2), 635–639.
- Stecher, L. I. (1916). The effect of humidity on nervousness and on general efficiency. In R. S. Woodworth (Ed.), Archives of Psychology, No. 38, pp. 40–65. New York: The Science Press / Columbia University. Retrieved from Google Books.
- Telles, S., Hanumanthaiah, B. H., Nagarathna, R., & Nagendra, H. R. (1994). Plasticity of motor control systems demonstrated by yoga training. Indian Journal of Physiology and Pharmacology, 38(2), 143–144. Retrieved from http://www.libraryofyoga.com:8080/bitstream/handle/123456789/60/II.1994.15.pdf?sequence=1.
- Treadwell, E. (1960). The effects of depressant drugs on vigilance and psychomotor performance. Experiments in Personality, 1, 159–196. An alternate publication can be previewed through Google Books.