There are several ways to approach it, but the simplest approach is to rank all of your players from first to last in terms of how efficiently they should score against a given opponent. (This is based not just on your players' abilities to score, but what kind of defense they are facing and who will most likely be guarding them in the game.)
Once you've ranked them, give a share of 1 to all of your weakest scoring options (freshmen, guys who are weak at PE and/or LP, guys who are facing the opponent's best defensive player, guys who have big SPD and/or ATH disadvantages against the players who will be guarding them) and then increase the share as the quality of the scorer increases. For me, if I am running a ten-man rotation, I'll typically have two to four players with shares of 1. Then maybe a few 4's, a couple of 8's, and maybe a 16 thrown in. Note that the total does not have to add to 100; the proportion of shots a guy takes is roughly based on looking at his distro share out of the total for the five guys on the floor at any given time (I say roughly b/c there are always things like fastbreak layups and tip-ins from offensive boards that don't follow the distribution). So if the five guys on the floor have shares of 1, 4, 4, 8, and 16, then the guy with the 16 should take slightly under half the shots (16/33).
The other thing that I have been doing forever (I think because it was mentioned in a dev chat or maybe a forum thread?) is making sure that no one player has more than 30% of the team total distribution. Somewhere along the line (again, can't remember where I saw this), I got the impression that if you went much higher than that it would mean that the player would be forcing shots and his efficiency would drop. So what I do if I have one guy who is by far the best option is to give all of the other players their shares, then give the top guy a share where he has just under 30% of the team total. (I don't know if that is an unnecessary limitation, but I keep playing that way because we all know that trying to change how we do things is bad, LOL.)
Hope this helps! - feel free to sitemail me if you have more specific questions, or just ask here so others can chime in as well.