How can I think on my feet better?

I participate quite often in class. Usually, I take time to formula a thought before even raising my hand. Once I am called on, I will just say what I already planned to say. Some say that I sound overly logical and mechanical, but my style has been working for me so far.

However, my style is weak in situations where I have to think on my feet. For instance, I tend to use a lot of fillers (umm….uhh…you know…, etc) when I do impromptu speaking.

Any way I could think on my feet better?