We have been keeping a seperate credit card account just for online and phone business. If I remember right the idea came from Kim Komando and her newsletter. The idea behind it is that if you ever have it compromised you can kill it, and keep your pocket cards active.
It makes it real easy also to keep track of your online purchases. Just recently our online card started showing problem purchases, and very quickly we were able to isolate it as fraud and have Visa destroy the number. That afternoon I was still able to buy gas.