Groin punch always works for me.
I favor the knee to tailbone method when I am in someone's guard. I try to emulate a pyramid in my preferred crouching method. I have a wide base, sitting on my heel with my back leg. The back leg is close to 90 degrees apart from my front leg. I hold my elbows to my sides very tightly and keep my arms bent to avoid being stretched. My elbows are just behind my front shin with my hands just in front of the leg. My chest is almost resting on my front thigh, and my chin is close to my knee. I am crouched low. If done properly there is nothing your opponent can get a hold of to manipulate. If my opponent tries to climb up with his hips all he can get a hold of with his legs is my head. I can easily come up under his knees or ankles with my hands to set up a black belt or similar pass. The most important thing to keep in mind about this style is to keep your elbows tucked in to prevent exposure to any number bad positions.