I give my her the tooth paste or her favorite doll to hold.
I ask my son, "what did you eat today?" then I say "oh I see some cheese in there"(or whatever his response) Then I try to brush out the 'cheese' and try to get him distracted by thinking about what things are in his teeth that we must brush out. If he tries closing his mouth and fighting it, I say "wait! I see some chicken nuggets!" or whatever. This sometimes helps distract him. If all else fails I put him in a headlock.
I'm struggling with the same issue. I've found that when I sing a song to him about brushing his teeth gently he'll sit still. I wonder if toddler gums are just more sensitive than we think.
I only have a 11mos old who is really good about letting me brush her teeth. We have been doing it since the day she came home from the hospital so it's definately something she is use to... occassionally she is unhappy about it... Since children really aren't good with cleaning all the surfaces of their teeth it is important that parents help them untill they have a good concept and motor skills though I do have to say even at 11mos when given the brush she does a decent job.....
I find that the easiest way to get the teeth brushed is to use flouride free toothpaste for babies and toddlers... and a brush with a small head... I lay her on her back on the floor and kneel next to her.... after making some silly faces and getting her to go AHHHH I get to work... if she's really fighting its easy enough to place my arm over her body to get her to hold still... this also makes it easy to tickle her which usually results in a open mouthed squele.... the laying them down makes it harder for them to fight and gives you a better view of their teeth...
I went to a bookstore and found the best book I could about the tooth fairy and read it to my kids. I told them about the great presents she brings for teeth. Good teeth. Not rotten teeth. Maybe my kids are easily bribed, but they ate it up (no pun intended), and have religiously brushed their teeth.