This is very rudimentary, and I'm by no means an expert on the NBA system, but here goes.
First off, the NFL Salary Cap for the coming season needs to be set at least 1 month prior to the draft because that's what I'm softly basing this from.
Take a percentage of the established salary cap, identify a "veteran cap" and then allocate the percentage solely for rookies. (i.e. 2009 veteran cap=90 mill, 10% rookie cap=9mill) That 9mill is what you have to spend on your draft picks, which would be "slotted" in the first round. That slot system would also be based on the overall salary cap depending on which pick it is.
This would also solve the "signability" issue. Look at MLB, where the worst teams don't always get the best entry players because they're represented by a Rosenhaus or Boras. That really defeats the whole intent of basing draft order on the previous season's record.
Also, throw a clause in there that if you're drafted by a team, you're locked into that contract for 3 years whether you like that organization or not. That would prevent incidents such as the Elway/Colts and Eli Manning/Chargers from ever happening again.
I'd need a bit more time to put something more detailed together, but hopefully you understand my opinion for now.