I found this yesterday while looking up draft entry rules. I advise hitting page down twice with your eyes closed prior to viewing, but it's a good read.
This particular "school" has sent a lot of players into the draft early over the past fifteen or so years, and has probably got a pretty good idea of the process. I couldn't find Michigan's version, although my search wasn't exactly thorough.
Basically, the following things end your eligibility:
1. Being represented by an agent.
2. You or anyone you've ever met including the cute chick from chemistry getting a gift from an agent.
3. Missing school to tryout with a pro team. (That one sounds like propaganda).
4. Trying out for an NFL team over a period greater than 48 hours (lots of windsprints and standing around in compression shorts...) that you don't finance.
5. You don't withdraw from the draft prior to 72 hours after the NFL's early declaration date. This year, that was on January 15th.
I know this has been talked about some here, but I wanted to get rid of any speculation. Donovan is out. Graham is out--although I'm still holding out for a miracle extra year there. Maybe he can change his name and be a Freshman?
...this question, -1 for the source even if that's where the Google machine pointed.
[edit: +100 for the fine Brandon Graham clip.]
LSA '89 - MBB Natl Champions, Big 10/Rose Bowl Champions | @MGoShoe