A few rules I follow:
1. When trading draft picks, don't use Madden's valuation system. Instead use a draft pick chart like this to determine value:
http://www.profootballtalk.com/draft/draft-trade-chart/
Notice how difficult it is to trade up under those values.
2. When trading your players for draft picks, undervalue your players heavily. While that solid veteran player may be worth a first day pick Madden, their value isn't anywhere near that in real life. There is not set formula for doing this; just pay attention to what NFL teams are willing to give up and try to follow that. More often than not, I would recommend just cutting your aging players if you don't need them anymore if you are aiming for realism. For example St. Louis couldn't trade solid veteran players like Orlando Pace or Torry Holt for anything; Pace was just cut and Holt appears to headed in the same direction.
3. When trading, make sure the trade makes sense for both parties. Madden's logic isn't perfect, and you may need to look at the roster of the other team to make sure that the players/picks improve their team. If the players don't fill a direct need, significantly reduce your compensation or find a new team to trade with.