I heard once if you try to pitch your voice lower it helps, but I've never remembered to try it.
Definitely.
First thing to do is make sure your lungs are less than half full of air. If your lungs are too full of air, you'll have to close your throat and strain (which is bad). To bellow properly, you want your diaphragm relaxed and the back of your throat wide open, which means your lungs should be at 50% capacity or less.
After that, tuck your chin low, open up the back of your throat, and bellow something like "GIDDY-up!" over and over. Lower your pitch each time you bellow until you can't go any lower. Do it in front of a mirror over and over until you find a level where you're loud yet comfortable at the same time. Imagine you're a drill sergeant counting cadence for marching troops or bellowing orders at them.
I was a sergeant in the Marines, and I've shown squeaky little females how to bellow loud enough to be heard a quarter mile away. Again, the key is to keep your lungs at 50% or less, so that you can open the back of your throat wide open. Then pitch your voice as low as it will go.
As for people who are monotone, the trick there is to imagine that you're a used car salesman and to inflect like mad: "COME on down to Big Al's! We're giving away EVERYthing! The deals are inSANE!" Public speaking is showmanship. So you have to inflect like a carnival barker or a used car salesman. You'll feel silly, but if anything you probably won't be inflecting enough. Your audience really needs you to go WILD!
When you're prepping the speech, indicate in your notes which words or portions are to be inflected. Type them in caps (like above) or in italics. Then practice the speech that way.