It is almost the same as you do when driving a car or truck, but the brakes. Try to remain calm, and immediately let off the throttle.Next, apply your brake, but not on the tire that's going flat; apply it to the good wheel and slowly bring your bike to a stop somewhere safe.
This is a great tip for any newbie. It is a shame more people do not learn about this stuff until after it has already happened and they have crashed.
Yes this dude next to me didn't learn untill after the fact, too late then. But guess he will be better prepared the next time. The best thing for anyone to do is take the saftey course, it will help a whole lot.