T.S.
N.,
I have 2 year old boy and he doesn't do that as much as he doesn't want to take off his PJ's or slippers or whatever for daycare. He has gone to daycare in his slippers before and had them on when I picked him up. Oh well. He also only wants to wear "dis coat" or "dis shoes" sometimes.
I think they are just trying to grasp some independence. So, what I've done is give him a choice, "Do you want the white shoes or the brown ones?" "Do you want peaches or applesauce?". Things like that. I give him a choice between two acceptable options and he thinks he's deciding.
With your daughter, it seems that she's taken this further than you would like. I would stick to limiting the amount of times that she changes. Perhaps you can pick out two shirts and two pairs of pants and say, "Today you can choose to wear any of these clothes when you want, but that's it." Maybe start with three of each to be consistent with your current rule and then drop down to two of each.
Hopefully, it will be a phase (the slippers were with my son) and she will grow out of it. I do like the idea of giving her a doll she can change 100 times if she wants!
Good luck.
T.