Well, it depends if you want simple or if you want things really fair.
If you want simple you can do one of:
* No players can be kept that are drafted before Round X (4, 5, whatever).
* You can keep anyone but you have to forfeit the draft pick of any players you keep, so if you keep your 1st-round draft choice, you have to give up your 1st-round pick in this year's draft.
If you want things to be really fair, you can do things that are more complicated.
* You can do a system like the second choice above, but you can keep increasing the round that a player counts for the longer you keep them, so you could (for example) increase by 1 round the first year they're kept, 3 (additional) rounds the second year they're kept, 4 (additional) rounds the third year they're kept, and then five (additional) rounds every year after that. And then if their round goes to or below zero, you can't keep them.
* You can do a system where you do straight increases, say 2 or 3 round increases every year, but you can only keep a certain number of players from every draft range... something like 1 player from rounds 1-3, 1 player from rounds 4-7, a player from rounds 8-11, and 1 player after round 12.
You can get even more complicated than that (I developed once a player value system for someone) but those would be the most common ideas.
So it depends on what you're looking for.
0-3 to 4-3. Worst choke in the history of baseball. Enough said.