Do you mean burdocks (the big round ones), or the little tick-seed things?
If they are the big burdocks and you don't want to just cut them out, then you are in for a long session of removing them. The best way is to tackle one matt at a time, and just pull small bits of hair out at a time. A knitting needle can be helpful (that's how I tackle the horse's tails).
The little tick-seeds can be combed out.