I've also seen (heard of) leagues using a draft round system to filter keepers.
Players taken in the first 2 or 3 rounds (however you want it) can't be kept.
A player adds 2 rounds to his draft position each year if he is kept.
Once that player would be in the first 2 or 3 rounds, he can no longer be kept.
Priest Holmes, Ahman, Moss, Portis, SA, etc. (first round picks cannot be keepers.)
Julius Jones was taken in the 10th round this year. He can be kept for next year if you want, but he will count as your 8th round pick. (He would be able to be kept for 3 years before he would be a 2nd round pick and have to be thrown back into the pool)
I think this is cool because it puts a lot more of the emphasis on taking chances on rookies and back-ups rather than current pro-bowlers. (I think it's more exciting to pick McGahee, J. Jones, S. Jackson, Roy Williams, Fitz., Roethlisberger and watch them become studs than to just keep Manning Ahman, LT2, Portis, SA, bla, bla, boring)