Burnish the tape as best you can, with a toothpick or similar stylus (hat-tip to you, Pantherman!)  Or whatever masking material you use (eg, vinyl masks).

Another trick some use is to apply a clear coat over the masking to seal those edges and prevent paint from seeping under the tape.  I use Future, for example, and I have tried applying it to the edges of the masking.  It worked OK, but I've never had much of a problem with paint seeping under the masking, so I don't do that every time.  When it has seeped, I've used a toothpick to scrape away the paint.