Best Brushes to For Fine Fur.

My sister's cat has extemely fine fur with a very thick undercoat. She keeps him trimmed from May-Oct but lets his fur grow in over the winter months. Is there a particular brush that helps to prevent matting. He seems to get mats really easy on his tummy and under his front legs.
Any suggestions would be helpful....

Ducky and Easy both have very baby fine hair. I use the FURminator to keep their hair brushed out, but around their arms and on their tummies, I use a brush just like this one .....
I find that the slimmer build gives me more control over brushing.
http://www.amazon.com/Boar-Bristle-T.../dp/B000LGHVPK

Thanks very much I will send the information along to her. Hopefully it will work for her kitty (Brady) as well.