Build up your mileage gradually - add 5 - 10 miles a week. Maybe you felt dead after your metric century you did, but do a few more 50 mile rides, and then a 60, then a 70, and then 60 won't seem so bad. Plan one long rider per week and 3 days or so of intervals or recovery riding in between.

If you pick the event you want to do, then make yourself a training calendar working backward from the century ride. If your long ride about 2 weeks before a century is at least 80 miles, you should be able to do the century without too much trouble. So work your way back from that.

Remember to pace yourself - ride your long rides at a pace you feel you could sustain for 100 miles.

And like Veronica said, be sure to eat and drink! Lots of riders (including me) use sports drinks instead of water for extra energy.