Without being too anal about it, just use the 70% stuff. Apply it to a cloth and rub the goop off the cpu. Try not to get any liquid on the motherboard. Repeat the cleaning process using a clean part of the cloth until all the paste is gone. Let it dry completely and then apply the new paste.
I just ordered some special CPU cleaner along with my new thermal paste, no idea how good it works but it seemed a good idea at the time. It was called Akasa Heatsink + CPU cleaner or something, I'll tell you how well it works once I've used it.