Okay, you have four options here. Option 1 is to whisk faster than the bowl spins. Not very practical, I admit. Option 2 is to get a bowl that has some sort of rubberized bottom. They make those. They sell them for kind of a lot. Option 3 is to get some of that puffy stuff for lining china cabinets that comes on rolls next to the contact paper at the store. Just cut a square of it and set your bowl on it. Option 4 is actually my favorite. Take a kitchen towel, get it wet, wring it out and then make a little nest for your bowl to sit in. Just form the towel into a circle that's a little smaller than the bottom of your bowl, and put it on your counter next to the stove (or wherever you're tempering your eggs). Then snuggle the bowl down inside its little nest. Now it won't go anywhere. And if you happen to get a little overzealous with your whisking, whatever whisks out of the bowl will land on the towel. Yay!