Masks

Frisket film can, probably, be used, too:

http://www.grafixarts.com/products/frisket-film/

It is made of polyesther and polypropylene and commonly sold in art stores. It is attached to smooth backing paper and comes in rolls. You can easily cut it with an X-acto blade tip to any shape without cutting through the backing paper. The masks are reusable and won't lift the paint. Airbrish artists use this film all the time. You can use it with acrylics for sure, yet enamels or lacquers may not be compatible.

What are you masking, Bakster?  It usually takes different techniques for different projects.

For some of the tricky parts of USS North Carolina in 1/700, I used some poster tack.

Sometimes you can combine masking tape with poster tack.

For planes, I've bought aftermarket masks, then traced around the patterns, made a photo copy, and taped the pattern over masking tape.  Then I cut out the pattern with scissors and apply it.  I've found that sometimes the purchased masks don't stick too well, but regular, cheap household masking tape will do OK.  I usually buy Duck brand masking tape.  Below, I used the old Eduard Spitfire masking set, which I don't think is available anymore.

If there's no aftermarket masks available, you can use a painting guide or profile to enlarge a mask to the size you need.

You'll probably receive other suggestions, too; but these have worked pretty well for me.

Good luck with your project!