Good luck! My son is 9, and I still have to remind him to brush his teeth. In fact, the dentist said it's pretty common.
Just help her brush as you can. When my son was younger, it would sometimes help if i pointed out what a big boy he was and also brushed my teeth at the same time.
If your child likes to read stories or has other pre-lights out activities she enjoys, you might add an extra story or let her choose, or reward her with a few extra minutes ofanother favorite activity, and lots of praise.
Let her pick out a fun toothbrush and kid flavored toothpaste. You might try purchasing one of the kid "movies" dealing with the toothbrushing and watch it with her.
Also, mention what happens to your teeth, in simple kid terms, when you don't brush. They turn black, rot and fall out.
If she's learning to count, make it a game of singing to 100, with one number for each brush stroke, that kind of thing. My son especially liked that. Or make up silly songs and let her be messy, having a foamy face from tooth brushing.