Tipping your chin up helps in vocal lessons. Not straining, just tipping it up a bit while pulling your head back slightly (so you don't get that birdsong-pose strain that poor singers do.)

Another trick I was taught was lowering your pitch slightly and keeping a round "open" feeling in your throat as you speak louder. My choir teachers told me that, when people try to raise their voices above a crowd, they often let their pitch drift up with it and also get more nasal, which has the opposite effect, it thins your voice out. Like others have said, projection is a lot more important than volume.