I drink caffeine in the morning, but I really need to stay away from it after that. If I drink coffee later on in the day (or just a lot in the morning) then I won't be able to get good sleep. If I don't get good sleep them I won't be able to concentrate.
Working out also really helps me fall asleep at night. I do some weight lifting during the day at work, and some cardio at night. I also don't try to force myself to get to sleep, I try to make sure that I will be tired by bed time by burning off any remaining energy during the evening so I'm tired when my alert to start winding down goes off.
But good sleep is absolutely the key. When I get bad sleep, especially consistently bad sleep I can't concentrate.