I think allowing for some salary retention during the season a player is traded would encourage some shopping at the deadline. Good teams can supplement their title push by getting talented players at a salary cap discount while others can aid in a rebuild by sacrificing some cap that season for draft picks or young players. There may have to be a limit to prevent superteams (like only a certain amount of payroll can be covered by other teams through retention), but it's an idea worth experimenting with.