Besides diet and sleep, a specific possible factor may be a low or deficient level of Vitamin D. If you suspect yours may be low, e.g. from not enough sun, consider getting your 25-hydroxy levels of Vit D checked. Supplementing with around 2000-5000 IU of Vit D a day could boost your immune system significantly if that is a factor.

Avoid Flu Shots, Take Vitamin D Instead by Donald W. Miller, Jr., MD

http://www.lewrockwell.com/miller/miller27.html

Featured Studies: Influenza and Vitamin D

http://www.vitamindcouncil.org/science/research/vitamin-d-an...

H1N1 Flu and Vitamin D

http://www.vitamindcouncil.org/newsletter/h1n1-flu-and-vitam...