Recently I have been getting finger cramps after longer painting sessions when using smaller brushes. I phoned art supply shops and googled for possible solutions. My thinking was that if i increased the diameter of the brush it would help.
I found these foam sleeves in a stationery shop. I think they are for going over pencils to make it easier for little fingers to grip. They cost very little and solved the problem. In fact, they make the brushes very comfortable to use.