Competence in unarmed self defense takes practice. This can be effectively done with a decent practice dummy (you can even build one yourself), or a partner, though I would only recommend a partner if either he/she knows what he/she is doing or you are both operating under the direct supervision of someone who does. Otherwise people can (and probably will) get seriously hurt.
The first and best self-defense advice is: Be aware of your sorroundings. Don’t wlak into dark alleys, take stock of anything suspicious or out of place, be sure to have your keys ready, walk with groups of friends, etc, etc. This is your first and best line of defense. It’s what has the best chance of saving your life.
Secondly, always look to get out of a situation quickly and efficiently. The guy wants your wallet? Throw it and run in the opposite direction. Call attention to yourself, NEVER get in some perps car. Better to run and get shot, your chances of getting away alive are probably higher.
Now, when all else fails and you get into an altercation, you must decide immediately if the guy means business. If you are in fear for your life it’s time to switch on kill mode and go all out on your defense. There is no hesitating, there is no playing it safe. You’re out to incapicatet your opponent and get away quickly (never stick around unless you are SURE things are safe, you never know what will happen next. His buddy might come to help with a weapon, he might get up, and go to his car to get his gun, etc, etc).
The mechanics of hand to hand combat are simple, but since the interactions are so varied it really does take a long time and skill to master. Your best bet is to attend a practical self-defense class. Tae Kwondo and other similar ‘martial’ arts are no good, specially for your needs. You need to learn the basics only. A few throws, a few strikes, a few locks, and a few escapes. That’s it.
My personal recommendation: There’s a lot of practical martial arts schools who author books and videos on this topic. My own interests lay in historicla medieval martial arts, but I’ve gotten to try out some material from more modern western martial artists. I like Pete Kautz of Alliance Martial Arts (google them) as most of his stuff has a good grip on what is martially sound and what is not. One place to start with them I’d recommend is their Medieval German Unarmed Combat DVD demo.
It shows the basic three wrestlings of the german system (dei drei ringen) and some strikes, how to fall, and how to practice safely.
Hope that helps. And do stay away from dark alleys!