My three-year-old daughter and I are going through this battle right now as well. And it seems like she always knows when I'm trying to be on time somewhere, because that's when she's at her worst! :) I have found a few things that help, at least to get us where we're going reasonably on time.
1. Distract her with a side issue.
Since she's keen on battling about *everything* right now, I often give her a side issue to battle about, while I quietly go on with the real problem. For example, if I'm desperate to get her clothes on her, I may distract her with a discussion about what we ought to have for breakfast. "Toast?" No! "Cereal?" No! Meanwhile, I am quietly helping her to step into her pants and put on her shirt.
2. Let her win sometimes.
Some things just aren't as important as others. There are plenty of times when what my daughter wants and what I want differ, but when it really won't hurt anything to let her have her way. After all, we want our children to grow into responsible, independent adults, and that means they need a bit of independence now. Sometimes that means that I take my daughter shopping in some outragious outfits, but big deal. As long as she's not trying to wear shorts in the snow, it probably won't hurt anything. That way, I can save my strength for the battles that really do matter.
3. Find excuses to praise her.
This growing up thing is hard work, and even when our children are fighting with us, they still need the reassurance that they're doing okay and that they're loved. Of course it won't always seem like it's helping--sometimes I'll tell my little girl that she's pretty and she'll say, very obstinately, "No, I'm cute!" Still, I think that it balances out the times when I need to be firm and set down the law.
4. Take an occasional holiday
I know, easier said than done, but if you can manage it, do it! Last week I had to appear for jury duty, so my husband and a babysitter got to take our daughter. That day was a godsend! I spent all day happily reading my novel, and when I finally got home, my husband told me about how difficult she had been, and how hard it was for him to get to work on time. I just smiled.
Good luck.