I think the key to good td defense is having a good, hard sprawl, and having good BALANCE. There's a lot of technique as far as td defense is concerned but balance is really what separates the great from the decent. Watch Mirko Cro Cop vs Yoshida if you want an excellent example of great td defense/balance. Footwork is also essential, but I think a good, fast wrestler is going to get a hold of you the majority of the time. You need to be able to really drop your hips and drop all your weight on them. The key to a good sprawl is really weight distribution, balance, and speed. You have to be able to sprawl on cue, rest your dead weight on your opponent, and keep your balance. It's also important to try and get an underhook, especially if they only grab one of your legs.
What a stud
The power of Aleks, his small baby and pimp hardcore dogs has granted you useful insight, my friend.