I've use parafilm-m with great results. The truth is, unless you use factory cut out masks, your best technique to masking canopies by hand is.
1) Patience
2) A steady hand
3) The sharpest exacto you have
4) Practice (probably the most important one on this list)
A layer of "Future" on the canopy (before masking and painting) also helps (a little) if you goof up. It gives you a very very small barrier between the plastic and the paint.