Clamp your miter problems away.
I stumbled across a link that lead me to Clam Clamps . It is a sweet little clamp that will clamp... well... miters. I have never seen anything like it before, but I am sure it would come in handy. The price is a little high at $75, but if it really does what they say it can do, then maybe it is worth the price. You also have to look beyond trim work. This bad boy looks like it can clamp any wood that has a 45deg miter. For me, this would really come in handy for joining columns together.
They also have a video showing just how much clamping pressure you can get out of the clamp, so be sure to check it out.