i had same problems too. but i've managed little bit to overcome it. try taking caffein rich stuffs before your speech like chocolate and coffee. that's helps to lower your heart-beat. another problem of mine used to b sweating a lot from forehead and palms. for that try taking deep breath after every two or three sentences. short-breathings cause u to panic and sweat. go slow rather than explaining too fast. don' hurry.

another, don't look at ppl if u r not feeling comfortable or choose one person with whom u feel comfortable. look straight ahead in the wall just above the audience and try to make instantaneous eye-contacts with ur audience.

if they laugh, laugh along with them. make fun of urself saying something like " that was such a fool of me...hehehe..." before they speak something.

and most important, choose the topic u know the best.