Hmm... My daughter is about the same age and I think she partly started dressing herself out of frustration because SHE wanted to wear something fancy and I'd pick practical things for her. lol. Does your daughter like picking out her own clothes? maybe see if you can set a few things out and then let her pick something herself. If she needs to work on the mere act of pulling things over her head, pulling on skirts/pants etc, maybe she like having some dress-up clothes?
I also find I can sometime get my daughter to do more on her own if I split up the tasks a bit, as in "you can get this shirt on while I go and get you a brush and hair clip" so she knows you are busy helping her in other ways and she needs to do her part in order to be able to move on to the next step (like going out somewhere, etc.) I just have been doing that lately so I don't end up the mommy waiter at meals (when she asks for juice, I have her get the cup while I get the juice or something, so we're working together and it isn't all me)
