Use only egg yolks, and make sure the temperature isn't too high. Egg yolks have a higher coagulation point than egg whites, and hence you have more room for error when using only egg yolks. And the temperature - well, eggs scramble at high temperatures. But you don't want it to be too low either, since the heat melts the cheese and helps make the sauce creamy. To keep the temperature just right, I make sure my pan and bacon has cooled down a bit and then add hot fresh-from-the-pot pasta to it, immediately followed by some pasta water and the cheese mixture.