Masking options
Liquid latex
I use this as my primary masking material for irregular shapes/edges (W&N\'s Watercolour Masking Fluid). I have had paint-lifting problems in the past every now and then - one time peeling off large areas of paint so I was not a happy camper as you can imagine - but it was my fault as I\'d messed with the paint recipe a bit.
If you use a good primer, on a clean figure, and your paints are decent quality it\'s not much of a problem but touchups are a necessary part of this method, so you have to get used to it. It is generally a good idea to remove masking fairly soon after you\'re done but I have left latex mask on for a couple of days during an involved spraying job - doing the face after the rest of a bust was painted - and it wasn\'t any different to remove than if it had been only a few hours old. This might very well vary from paint to paint so do experiment for yourself.
I often spray before it has fully hardened (i.e. there are opaque spots where it\'s thicker) which doesn\'t seem to make any difference in my experience. I\'ve even been done with a spray job and still had wet spots here and there when I peel the mask off, in the corner of an eye socket for example or under an arm.
Rough edges are hard to avoid entirely but one trick I\'ve learned is to apply the mask a little over the line you want and then when it\'s dry push the edge back gently to the correct position with whatever tool you\'re comfortable with, if you\'re lucky doing this you get a softer mask line, because the edge becomes raised, which can avoid the issue of a paint \'lip\' completely. I usually use a sculpting tool for doing this as they\'re on the desk anyway but a fingernail, colour shaper, a brush handle or even the side of a needle all work.
Other masking materials
I got some Parafilm M on the advice of Mario Fuentes and it really is special stuff, anyone who has done aircraft or knows someone who does will probably be familiar with it but it\'s well worth trying out for some masking jobs of figures too. This clings without a true adhesive so lifting of any kind is practically non-existent but be warned that it melts if you heat it so you can\'t dry paint using a hairdryer between layers (I learned this the hard way!) I got a full roll (250 feet) from a laboratory supply house, Pollardwater.com, which had the lowest price I could find at the time. By the way, there are other uses for this as it\'s a complete moisture barrier so it\'s handy to have around for things other than masking too which helps it pay for itself.
For hard-edged masking Tamiya masking tape is very good apparently and there are other masking tapes that might be worth experimenting with.
Don\'t ignore simple things like a damp piece of tissue, this can work for all sorts of jobs - aircraft modellers sometimes use this for filling wheel wells or cockpits before spraying. I\'ve started to use Glad Wrap to mask the bulk of a model for spraying because again it has no adhesive and it\'s cheap and easy to apply.
Einion