Is she a small breed? Small breeds often have dominance issues - mainly because being dominant has been breed out of many larger breeds.
Anyway, it's not that tricky to assert dominance over a dog. There are two main things I recommend.
Firstly, on a regular basis, you should gently but firmly pin her on her back and put your hands on her neck. Don't hurt her, but don't let her up either. Hold her like that for a little bit, then let her up.
Secondly, start giving her commands before you do nice things for her. For example, if you're about to feed her, tell her to sit, and only give her the food once she's sitting. (Make sure to use a command she understands, of course.) Or if you have a fenced yard she runs out in, make her sit before you let her out.