Suits

The most important thing to think about when it comes to the fit of a suit is the shoulders.  Since most suits are made to be sold off the rack, you'll have the hardest time with finding one that fits your shoulders properly while also fitting your arm length and body size.  This is because the shoulders aren't usually altered by most retail shops.  If you buy a suit that has over-sized shoulders, you end up looking like old Japanese daguerreotypes of the Samurai class, all puffy and goofy looking.  Conversely, if the shoulders are too small, you'll just be uncomfortable and you won't be able to move.  When you're trying the suit on, move your arms around while looking in a mirror to make sure that A:  you have an acceptable range of motion and B:  they don't pop up or slide around when you move.

Aside from this, just adhere to the regular rules.  You want a suit that fits you somewhat snugly and is comfortable.

The Fit Is The Thing

The fit of your clothes is more important than the design in many respects.  It determines how you wear them, how they look and can influence how you carry yourself.  If you spend the time to find clothing that is a good fit, it will pay off in the long run.