It's always difficult. If it's a dynasty league, then teams that are out of it can still make smart pickups to improve their situation for the following year, and tend to stay a little more active. Some sort of money-based league could maybe have small weekly pots that handicapped teams or something. We've toyed with the idea in dynasty of weighting draft lotteries to (very) slightly favor top performing non-playoff teams, giving teams the incentive to finish as high as possible. We've never implemented it in any leagues though.