I know this comes from bim's book of the obvious, but are you sleeping 7-9 hours a day?
Do you snore? Are you overweight? If you snore you could have sleep apnea which sort of keeps you from getting good restorative sleep, because you are constantly stopping breathing, therefore waking yourself up. Being overweight can also cause fatigue. The solution to being tired is rest, like haennie said. How about exercise? Do you get any? That helps with sleep, too.