I've been looking at a Fine Scale Modeling magazine this week and learned two interesting things:
1) For masking around curves, use plumbers tape.
2) For masking larger areas, use blue tack to lay the boundaries and then place plastic (sandwhich bag, saran wrap, etc.) over the blue tack. Use an exacto to press the plastic into the blue tack while it cuts. Paint and then remove.
These modelers have short cuts I would've never thought about.