Lucinda,
don't give up! we are constantly trying new tactics as our daughter's temperament towards teeth brushing changes. Here are some we've tried...
-sing while brushing (this is our favorite)
-both of you hold the handle together
-let her hold one toothbrush while you brush with the other
-have 2 brushes and let her pick which one to use that day
-take turns
-let her sit in a new spot while brushing (like the countertop or on your lap)
-lots of positive praise for a good tooth brushing session
...and yes, we've also resorted to :-(
-pin her down and do it while she screams (stay calm and don't get angry, just do it in a matter-of-fact way. *I promise we NEVER hurt her doing this)
luckily, we didn't have to pin her down too many times, but she got the message that brushing her teeth is not an option and now she pleasantly opens wide for us to brush.