How do you prevent touching up paint when you have masked off the area?

I watched one of the those decorating shows as they used painters tape for making squares on the wall,and as they peeled off the tape there were clean crisp lines with no touchup work they used something clear..but not sure what it was does anyone know what i could use ?How do you prevent touching up paint when you have masked off the area?
They paint a clear coat called Gesso sold in art supply stores a water based acrylic that you use before applying color just paint along edges of masking tape and will fill any voids under tapeHow do you prevent touching up paint when you have masked off the area?
Most of the time the paint line is not crisp and straight, they just pull the camera back so it is not noticeable
When you tape. the tape does not have to be clear. Blue painters tape is low tack and will not pick up the underneath paint when you peal it up.





After you lay the tape down, take the back of your thumb nail and smooth the edge down TIGHT so it will NOT bleed under the edge.





Peal it up when it is dry. That should do it.





Happy Holidays.