Brandon is right on here - but I think its worth mentioning that the #1 - squeeze & extend the lockdown should be the LAST thing that you do to elicit a tap. If you do this before pinning the guys knee to the mat with your shoulders than you will find yourself struggling - your upper body just won't be strong enough to pin the guys knee if you already have the lockdown squeezed & extended.
In my experience this is where most people go wrong. If you can pin the guys leg to the mat with your shoulders 1st (or as close as you can get) - then when you finally do extend & squeeze your legs you'll get the tap on all but the most flexible guys. You'll have the combined strength of your hips & both legs against just one of your opponents legs - that is a battle I'll take every time!
I actually hit this in my last tournament & it was my favorite win of any of my matches so far. I hope this helps...
http://www.youtube.com/watch?v=ZMnq9auUr_I