Preparing for the May supplemental draft is hard, if you look past the un-drafted players with the most At Bats and Innings. It might not be worth it. If you are in a perpetual league, of course, there is more of a chance of finding the next Hank Blalock. He was drafted in the May draft of one of my leagues a couple years ago.
To do this efficiently, you need only three tools. The first is Baseball America`s minor league stats (http://www.baseballamerica.com/Stats/index.html) or some equivalent. I would start with the AAA teams, then, if you had enough time, work on the AA teams. The California and Carolina A League teams might be nice, if you are a true fanatic. However, I`m sure, if you are that fanatical, you would probably overrate the high A league players by quite a bit. With these minor league stats, you are looking for players with enormous K:W ratios (8 to 1 gets my attention) and a decent ERA to match, as well, you are looking for players with an O+S of about 1.000 or higher for an outfielder or firstbaseman, .900 for a thirdbaseman, or .850 for a middle infielder or catcher. They must be every-day players not utility guys.
Make sure they are not already taken, by doing a search (Find from the Edit on the tool bar) on the player from your team`s website.
Then you read about them from your favorite prospect rating source.
Make sure you know how long he has taken to reach that level in the minors. Baseball Cube (http://www.sports-wired.com/players/) will do that, if your prospect source doesn`t.
Also, check on the A level performance of anyone else who has been highly recommended from that far down.
That`s it. By the way, there are no Hank Blalocks available in either of my perpetual leagues. There is one intriguing prospect with strong potential available from AA. There are a bunch of guys in AAA who are good candidates to get called up, but not necessarily any better than the top 20 or 25 guys available who are already up. And there are a couple more prospects, who might be fine long term choices, but they have such a long ways to go, they might not be worth the wait.
Good luck. To do well from Phase III or the supplementals, you need hard work and luck.