It's best if you start brushing near the ends of the hair & only work back towards the roots once you've got some of the end clear. Just keep starting further back up each section as the ends become untangled.
Is this as clear as mud?
It's a bit hard to explain.
If you begin near the skin it just makes things worse by pushing the mats into the length.
I find the best way to prevent them, once you've got rid of them, is to use a really good conditioner when you bath. I even use half & half conditioner & shampoo to wash her when her coat's really long. Then just give a good once over with a bristle brush every day to stop them forming. It's quicker in the end than having to detangle mats.