I'm actually a pretty competent public speaker. I like to keep my style relatively informal so to me there is little difference between speaking with one person or a large group. I find if you try and memorize a text you're going to mess up; because if you forget or go off track, its relatively hard to get back into your flow. If you just know the material very well and go off the cuff its much easier. You can even have a PowerPoint or cue card with a few points on it to maintain somewhat of a structure.
And in the same vein, when I finish speaking in front of a whole lot of people I always get a rush of adrenaline. Don't know if this happens to anyone else ...?