We've done a couple things after a public outcry over dump trades.
First, we moved the trade deadline up. More teams still in contention meant fewer lopsided deals.
Second, we implemented a "trade tax". BOTH teams involved pay 10% of the difference in salaries being exchanged. Example: Team A trades Manny Ramirez and his $40 salary for $5 Rocco Baldelli, both teams ante up $3.50 to the pot. It's not a deterrent, but the financial aspect makes the trade a bit more equitable.
We toyed with a rule that would require all players traded after a certain date to become mandatory keepers for the next season, but there was too much unknown about how that would influence things.
As long as both owners have intentions of returning to the league and are considerate of the keeper aspect, I haven't really seen too many lopsided deals that haven't evened out eventually, though! So don't go overboard on policing dump trades...