Let them listen to a real conversation or even a real speech. You can use video, podcast, etc, then let them analyse the speech (Imitating won't hurt)... I use BBC...
Let them have a simple debate. If you compare debate and speech: speech is harder since in debate friends can cope and help one another.
Pick easy topics. I call this debate 'stupid debate'. Fyi, I use this method all the time for my class. The debate is normally about 'stupid things' eg: Superman VS Spiderman, Contact lens VS Spectacle etc. And then pick one person from each group to come forward and summarise the whole thing. Note that this is similar to having a speech (in a sense).
After a while, they will get the grip of it and I'm sure they won't have any problem delivering a speech... One step at a time... ;-)