1. Do a plank every single day. Practice really makes perfect.
2. Turn the stop watch over so you can't see it. The closer you get to your time goal, the more you'll pay attention to how your muscles ache.
3. Read a magazine article or the Internet. Put something on the ground in your line of sight so that you can distract yourself with something other than planking!
4. Countdown backwards in your head really slow. Don't speed up your count!
5. Do it until it hurts, then hold for 30 seconds more.
6. Listen to your favorite music, average song lasts 4 minutes. It's all about distraction :)