The best thing that has worked for me is to exhale before I begin speaking. Before I realized this I had always started out with a lungful of air and it was just bad. I'd try to say as much as I could in that one breath and then gasp for another lungful and repeat. Once I started deliberately starting a speech with an exhale, it felt so natural and easy.

The other thing is with the ums. After finishing a sentence, I just pause to gather my thoughts and resume talking. One trick I've read in many places is to have a cup of water with you so you can take a sip if you need a couple of seconds to pause.