Stress - probably the biggest and most common factor
Booze - yes, it makes you tired, but at the same time it also fucks you up and makes falling asleep more difficult. The next day you feel like shit and all you want to do is sleep it off, which ruins your next night of sleep.
Coffee, tea or coke - I cannot drink coffee after noon without it keeping me up at night.
Eating large meals before bed
Stop using your bed for things other than sleeping (and fucking - not a...