I've done plenty of canopies as well as camouflage with plain old green or blue painters tape you can get at the home improvement stores. Lay it on your pants and pull it off a couple times to reduce the tackiness of it (watching out for lint of course). I've used Tamiya tape, but I'm not as thrilled with it because it's thinner and tears easily. It does have the advantage of forming over curved parts a little better. Recently I used Bare Metal Foil to mask an F/A-18 canopy. It tears very easily too, but you can burnish it down into all the curves and detail lines of the canopy. Then use a brand new #11 blade to cut the areas you need to paint, peeling off the BMF there. I may switch to BMF for all my canopy masking.
To me, the tool that is way more important than the type of masking agent is a good dose of patience. The job is tedious, but you have to go slow and careful to get good results.
Chris