1) Sleep 6-8 hours a night. No more no less. That length of time is pretty important, even though most people ignore it. Not only does sleeping more or less lead to sleepiness, but it can cause serious health and memory problems. Heck, it can lower your life expectancy more than things like being fat or smoking.
2) Sleep the same 6-8 hours every night. Yes, even on weekends. This means no more late nights, and no more sleeping until noon whenever you get the chance.
3) Always get up the first time the alarm buzzes (no snooze, no getting back in bed). Actually, if you follow #1 and #2, you'll usually be awake anyways by the time it buzzes. Get up immediately when it buzzes.
 
If that doesn't work, check into what Guyver01 said, you very well may have a sleep disorder. Caffeine, walks, smoking, affairs, etc all just try to treat the symptom, not the cause. And they do that poorly.
 
I sound like a depressing buzz-kill. But it works. You'll thank me later if you ever try it.