I use Akasa TIM Clean. A few drops on a lint free cloth, apply to the left over paste, leave a couple of minutes then wipe off.
But I think Isopropyl Alcohol is used alot from all the stuff I read, but I could never find anywhere near me that sold it. So I got the Akasa stuff from my local parts supplier the last time I was buying stuff.
I had an old heatsink sitting around so I took paper towel and some hydrogen peroxide. It cleaned it very well, but I don't know if it was necessarily good for it.
I used 90% Isopropyl Alcohol and a coffee filter. Got the stuff off real quick. Found at your local corner store in the first aid section and less than 2 bucks a bottle. Its basically rubbing alcohol for cuts. Besure to get 90%+, it will say on the bottle.
I've heard of people using nail polish remover. There are only a few things (to my knowledge, which is limited) to consider. Do not use non-acetone removers and do not use any with fragrance.
just to say, i used a acetone based nail varnish remover (that had quite a ice fragrance) to re-apply thermal compound, seemed to of worked so far, and now i have a sweet strawberry fragrance wafting from my xbox