Marianna Baer posts at Crowe's Nest on using distinct voices for similar characters. This is particularly important in YA books, where you often find teen posses, and children's books, where you often find lots of friends.
I find it helpful, myself, to cut down on the number of secondary characters as much as possible. Then, at least, you need fewer voices.
Training Report: I'd hoped to look at some work this weekend. That doesn't seem like asking much, does it? Well, it seems it was. I did, however, mow the backyard this morning. I had one stellar idea while doing so, which I hope I remember.