This can be such a pain! I have a 2 yo. We try different things all the time. Sometimes DD is very cooperative other times not. First of all, if you are anxious about it, it will make it more difficult so try to relax. Some of the things we do:
-give her a choice of toothbrushes
-ask which song she wants us to sing while we brush
-give choices about where we will brush-in the bathroom, bedroom, sitting, standing, etc
-let her brush my teeth while I do hers
-if she is really getting upset, I stop, give hugs or whatever and resume in a few mins
-if she is not cooperative at all, ask if she has sore teeth and then avoid the area she says is sore
-we race-daddy tries to do her teeth before I finish mine
We just try to be very flexible. I didn't want a negative association with brushing so at first we did a little each night and built it up to more and more time. But I also treat it as a regular thing that will get done, though it sometimes takes a lot longer. I found that trying to force it didn't work for us at all. It does get somewhat easier to "reason" with them the older they get. I will tell her no stories until we are done (as that's the next part of the bedtime routine) but I don't do rewards for it, because I feel it is something that needs to be done, and if I do rewards, she could just decide she'd rather have no reward and not brush. Not an option. Just keep trying different things. Good luck.