well, lay off heavy meals, drink enough water (not gulp gulp, but keep hydrated throughout the day).. lay off caffeinated drinks like sodas, coffee, tea, hot chocolate...
lay off cheeses (you rat!

) and be relaxed...
in your room do not do anything other than sleep.. do not even watch TV or anything... use it only for sleep...
dark room (duh!)
quiet...
and if you have an airconditioned room, keep your feet warm.. wear woolly socks if you have to...
before sleeping, take a long hot shower to relax you...
turn off your mobile phone and let nothing except the next day's alarm disturb you...