The radical way to deal with this is to disallow trading draft picks altogether... It may not be a perfect solution, but quite a few leagues use this rule in order to cut down on the problems you mentioned.
Another way to go is to shift from a standard draft format to an auction. That way, if you trade Peyton Manning and his 20 dollar contract, you'll have an extra $20 to play with in next year's draft. Auction leagues aren't perfect, either, but if you add in elements such as rising salaries and contract lengths, you do have a certain amount of checks and balances in place to prevent iffy trades.