My local discount store sells a range of 4 brushes for next to no money. 2 are wire and the other 2 are plastic.. The plastic are perfect for brushing off the worst of the mud and dirt after a bash.

Toothbrushes are ok to but I find the bristles to short.

I also use a old rag, canned air and GT85.
GT85 is a great product as after the oil dries it leaves a coating of PFTE on the car and stops any dirt from sticking to it. I coat all my new RC cars and parts with it before taking them out. That way most of the dirt just falls away and the rest the air gets rid of.