Wow! Thank you for all the kind words, gents! Truth is, I'm just Forrest Gump-ing my way through this. I've been standing on the shoulders of giants (Jayvee, AFFO$, rafalfett...etc)!
Great question, Booch! But kind of complicated to answer. In general, I've left masking on until all MAJOR colors and misting is completed. But, as you can see, the helmet is no longer masked at all, but I've continued adding details.
What I've tried to do is continue adding details, masking where necessary, but adding small amounts of masking here and there for fine details. I've switched out my airbrush needle from .5 (wider spraying area) to .35 (finer, tighter coverage). In addition, I've dialed down my air compressor from 20 psi to around 12. This means that a very fine stream of paint comes out of my gun, which in turn leads to more control. That means I don't have to mask much, because so little paint is being sprayed, and at a very low pressure. What THAT does is make your airbrush much more similar to a paintbrush. Very fine and controlled.
I've had to redo several sections, but knowing your air gun and your compressor certainly makes things easier. For the remainder of the paintup, I won't need to mask or tape off and add bags.
I'm sure that's a confusing answer. To simply, once the MAIN colors are laid, masking becomes less and less necessary.
It's HARD to fight the urge to remove masking between colors. If you can at least restrict that urge to the MAIN colors, you'll be fine.