fraggle

+1 lightest touch will do it. A lot safer than many methods - I started with te plastic razorblade but it wasn't tough enough. Used a real one and got paranoid. Found an awesome local guy and they came back clean as a whistle. Sometimes you just have to call the pros!

EspritS4s

I've used razor blades and green abrasive pads, by hand, for the tougher spots, such as piston tops. Tedious work though.

blown 87

Some times you can get away with it, many have, but it is never a good idea.

generally it is going to take 4-5 thousandths to not only clean a head up, but to make it flat and square, my rule of thumb is one thousandth per cylinder is what is going to take, a inline 6 will take more than a V6.
That is also how much I allow for being out of square.