How can I be more be more articulate?

ok basically i have a problem with speaking & getting my words to flow natural. I hate it because when im thinking, i can usually think of some clever and compelling opinions, but its when someone asks me "what do you think, gary?" and they put me on the spot, i get nervous and i have a bunch of things running through my head and its like they all wanna come out at the same time and its just a complete mess. i have had this problem for as long as i can remember and i think its because i have ADD (diagnosed when i was 6). when i go to talk to someone especially people i dont know and am talking to them for the first time on the phone or something, i get nervous & all comes out is verbal vomit and i start with the uhhs and the umms and because im doing the umms and uhhs i become consious of it and makes me MORE inarticulate. It has made me blow a lot of chances for different opportunities in life and luck with girls. also when i leave messages the same thing happens and i blow it. help!