It's mentioned offhand in some of the replies here, but one simple thing that will do a lot for you:
Eliminate "umm" (and all filler words) from your speech.
It's a devil of a habit to break, and for most people takes great focus & the cooperation of your peers (to point out to you when you slip) but it makes you sound sharper and forces you to think more carefully about what you say.
I wish I could eliminate 'like' from my conversation. I do try and make a conscious effort to reduce 'umm' and 'err' but someone 'like' keeps slipping into casual conversation.
I'm much better in professional settings and I have a tendency to pause when stuck, rather than fill in with filler words. Casually and socially I'm much more likely to fall back on useless terms.
It is a very tricky habit to escape. I wonder if you could build an all for that, that listened to your speech and buzzed when it picked up filler words?
Find a few friends who also would like to quit using filler words. Monitor each other. Have some keyword you say when somebody just said a filler word.
Eliminate "umm" (and all filler words) from your speech.
It's a devil of a habit to break, and for most people takes great focus & the cooperation of your peers (to point out to you when you slip) but it makes you sound sharper and forces you to think more carefully about what you say.