When you're selecting a martial art, the number one priority is the instructor. I took a shorin ryu class once - that style is pretty deadly if it's taught right but this class was certainly not. Anyone that gets out of the dojo I went to will get killed if they try to use what they learned on the street. Make sure the instructor knows what he's doing. Best way to insure that is to take a friend who's very knowledgeable in the martial arts along to spot any potential bullshit going on. A good test, though, is in the sparring. If they're stiffly delivering punches while their opponent "blocks" the punch, you've found a McDojo - run away.
The other important thing is not to lock yourself in to one art. A good blend of what works best from many arts makes you a much more well-rounded fighter.
Many instructors have moved to a mixed martial arts format so you don't have to seek other dojos. My personal preference is a mix of ryuku kenpo, kali, muay thai, and western boxing (with a few dashes of other arts thrown in for flavor).
The most important thing is to train as though you are fighting. Your real attacker isn't gonna punch like a classical karate student, so why should your sparring partner. Strive to make your practice sessions as real as possible. The only concession to reality that I make (other than the obvious of fighting with practice knives, etc) is to wear a gi - mainly because street clothes get torn up too fast the way my dojo trains and I can't afford to keep replacing 'em
