i think one of the better things to do initially (although some people think it's stupid) is to make some cardboard cutouts of the dimensions of a 42" and 50" screen. I made two cutouts and put each one up on the wall where I was going to place my plasma, and it made a huge difference in my purchase. originally i was going to settle on 42" screen, but after seeing the size difference on the wall, i ended up going with the 50" and am extremely happy I did. some walls will be too big or little for a certain size of screen, so having cutouts and putting them up on the wall will let you actually see how it will look and how much space it will take up. trust me, seeing them in the store and seeing them on your wall are two completely different things. keep in mind your viewing distance also...8' viewing distance for 42" screens and 10' for 50" screens. if you sit 7 feet from a 50" plamsa, you will not be happy and think the picture isn't as good as it is supposed to be, same thing goes for 42" if you sit too close or even too far and then think you can't see details in the picture (think of it as a movie screen in a theater....sitting real close you can see blurred images and fuzzy pictures, but sitting further back, the picture looks crisp and clean)
now on to mistakes:
1. make sure to run cables and make the holes in your wall BEFORE you mount the tv. after mounting, i realized i wanted to hide the wires and then it becomes a hassle to remove the tv, cut holes in the walls, run cables, and mount it back up (my wife isn't exactly supergirl so i have to rely on friends and when they can actually come by to help me).
2. don't settle on a cheaper brand only because it is cheaper..make sure to get the best quality brand/model for what you can afford. panasonic, pioneer, hitachi are all top performers and you will be able to tell a difference. i had gotten a philips (one of their first generation panels) and wasn't thrilled with some of the features and problems with the plasma. i am currently going to be replacing it with a panasonic 50px600u which is the top-rated plasma on consumer reports.
3. don't place the plasma on a wall that is opposite of a wall with windows (common knowledge as far as i am concerned though). you will get glare during the daytime, but if you do most of your viewing at night you are okay, but remember that football is on during the day on sundays :)