If you're left-handed, put your brush in your right hand, and if you're right-handed, put it in your left. It'll feel awkward and you won't be able to paint as precisely as you can with your dominant hand. This lack of co-ordination also means that you can't get into that automatic paint mode where your brain says "I know what an apple [for example] looks like" and you paint an idealistic apple rather than the one in front of you.