If you want to curtail tanking, you could also make things less predictable. 1st round flops, and less predicatability in the arbitration/FA signing period would be one area. As it is right now, I know I can pretty much control any player for most of 11 seasons (20 day waiting period for season 1, 3 more pre-arbitration seasons, 2 seasons after arbitration and a 5 year deal) meaning I can draft a star at 18, move him up to the big when he hits 22 and have him under control until his 33, at which point the decline starts just as he's hitting FA for the first time.
Why not have some guys refuse to sign a LTC at any point, or perhaps only a LTC that will eat up some of the arbitration seasons. I think its the predicatability that makes tanking attractive, you know your #1 pick is going to be a star, rack up 3 or 4 of them, plus a handful of top IFAs and you have a top notch core together in the bigs for 10 seasons.