I use finger but I wrap the finger first with thin plastic film (I don't know what you call it - the type you wrap food stuff in). The film stretches somewhat so it is easy to wrap it around finger so that the film has no wrinkles on it.
This way there is no risk of grease and I can get really thin (like almost see through) and even layer of AS II (possible finger prints will disappear under clamping pressure). This doesn't work so well on large uneven surfaces like northbridge or GPU without lapping, but very well with CPUs.
Normally I first clean everything with acetone, then smear enough AS II to HS and wipe it off, then to cpu, take excess off with knife or credit card and finally tap the rest gently with finger to smooth even surface (plastic film takes some of the AS II, if there is still plenty of it on CPU, I change to clean film).
