Grace'sDad

This works for me although it's a little unorthodox and will cost you a couple of cheap tubs of addy:

Spread some cheap and nasty tubbed adhesive onto the wall with the flat side of the trowel - just enough to cover the paint.

I find that if you leave it about one hour, then the paint will scrape off easily and cleanly using a sharp decorators scraper - the jumbo ones with a removable blade.

It's a damn site cleaner and easier than trying to get it off mechanically.

I hear; I forget
I see; I remember
I do; and I understand.