First off, we will need some additional information... How many teams in the league, roster size?
If you are in a 12 team league I would not have more than 1.. 2 keepers max. One of the most fun and exciting features of fantasy football is the draft. Don't ruin it by having too many keepers. Since you are in an auction league I would also recommend that those keepers have a premium raise to next year's auction draft were their value would be deducted from next year's salary. In other words that keeper doesn't come for free.
For example... Team A paid $43 for Foster in the 2012 draft and decides to keep him in 2013. That team will get charged $48 off his budget for 2013 ($43 from last year's draft plus a $5 premium... You decide the premium scales, might be a %, or a set number based on value). With a system like this it makes your owners think about "value" and the "overall strategy" instead of just keeping a stud player just because they drafted him the year before. In this example a stud player might be overpriced and it might be a better (cheaper) option to try to throw him back in the auction pool. Also give your owners the ability to opt out from keepers and instead enter the draft with a full budget.
Finally, you need to establish a rule as to how many times/seasons you can utilize the same player as a keeper.
Auction leagues can be confusing as it is, adding a keeper component makes it even more challenging but a good balance of rules can make it rewarding.
Best Damn Fantasy Football League