Sometimes when Joey wears clothes a lot, he mats under the arms so before I bath him, I use Cowboy Magic detangler. It really works well in getting out the tangles. I also use extra conditioner in this area, after shampooing and this seems to help. He hasn't matted in a long time. Here's a link that gives tips on keeping a bow in a puppy hair.
Casual Way to Put Dog Bows In