Once the edges are masked, I come back in with wider strip of Tamiya tape and finish masking the areas to be covered. Be really careful and make sure that you don't leave any micro-gaps where paint might sneak through. (Again, this is experience talking here!)
When masking in areas where I want a really sharp edge I always burnish each strip of tape as its laid down, then after everything is masked, I go back over what I've done and burnish the entire area with vigor and enthusiasm. This will seal all of the various layers and help prevent that dreaded overspray from seping under the ONE loose flap in your masking job! Remember, you are building a series of miniature models that you will eventually assemble into the finished model. This kind of attention to detail is a pain in the ass, but if you want to continue to improve and take your modeling to the next level, take the time to do the little things. Believe me, it WILL pay off!