How do I keep paint from seeping underneath the painters tape?

There was recently a program on t.v. which I was not paying close enough attention to and they told of something one should apply to the edges of the tape, AFTER IT WAS APPLIED TO THE WALL, but I don't know if it was just before or after applying the paint. It would prevent the paint from oozing underneath the tape and would give a very sharp division after pulling the tape off, rather than looking jagged & crappy.
I believe whatever it was they applied with a brush.
Can you help?

don't use regular masking tape. use 3M blue painting tape and roll it on before painting.