I know there are a fair few of us who have trouble getting any kind of tooth brushing cooperation and we often have to resort to the 'mum knows best, hold him down and just do it' approach
Well I was employing said approach and started tickling DS at the same time. It worked brilliantly - he laughed instead of cried and his mouth was open for me to have a brush! I kept up a commentary about the toothbrush tickling his teeth and where my other hand was going to tickle him next and it worked really well. He didn't know whether to laugh or cry, but if the crying came back through the laughter I'd blow a raspberry on his tummy and start tickling again. This isn't the easiest technique as there's a lot of wriggling and writhing going on but we had a lot more fun than normal!
Actually, this would be good if there were 2 of you, one to do the tickling and one to brush.