Visit the venue ahead of time to check things out. Take a friend along, and have them sit at the back of the room. Speak in your normal presentation voice and make sure that you can be really clearly heard. Remember, when the room is full, your audience will bump up the ambient noise quite a bit.
Consider using a microphone if you have a larger audience. Don't be shy about using a mic. It's no reflection on you if you need some help in a large room or with a large audience. Everyone does! You can get an unobtrusive lapel mic, so your audience will not be focused on the mic. Get a cordless so that you can move around without being tethered to the mic.
If you are using a mic, all the more reason to get to the venue early and check that the equipment is working properly. The last thing you want to do is deafen your audience with feedback!