I'm in the middle of a 12-team keeper draft, but it's my first one, so I'm hardly an expert. But for what it's worth...
First few rounds, I've drafted the best available, regardless of position. I admit that I have been swayed slightly by age - I took MCab in round 2 (pick 18), slightly earlier than may be expected. But then, I took Sheffield in round 3 (pick 31), as he was easily the best hitter available, and worth it - I'm aiming to win this year as well as the future!
One thing I've noticed is that prospects seem to be drafted earlier than I would take them, even in a keeper. Examples:
D. Mcpherson - round 9
D. Wright - first pick in round 6
J. Bonderman - round 11
S. Kazmir - round 12
Nothing necessarily wrong with these picks, but they're all earlier than I would consider them. So if there is a prospect/rookie whom you really want, you must be prepared to draft him early, otherwise someone else will grab him.
Probably not much help - I'm learning as the draft goes on - but it may give you an idea of how this draft is going!