1. I think sometimes we make martial arts seem like some super-human ability. Lcukily a lot of the time attacks are by drunk thugs, and so just the exeperience of training in physical body contact from attack situaitons can help us (which applies to basically any martial art).
2.Also the martial art is only as good as the martial artist.
3. No two situations are the same. I know of someone who had trained in aikido for several years and was hit from behind in the spine with a crow bar; he fell down and couldn't stand up again for about half an hour (lucky he wasn't paralised). No-body is unbeatable.
4. Constant practise in whatever martial art helps you to react instinctively to situations. Most of the time even a poor technique can be carried off because the opponent is unsuspecting. As mentioned previously in another thread, there is a lot of psychology involved.
5. Some martial arts of probably better for different situations (e.g. aikido is often thought one of the better ones for armed attacks, but is harder against people who box/jab).
So basically, to be effective in more and more situations, train more and more and you have a better chance of beating the odds.