One of the big quality areas that set painters apart is their ability to paint straight lines. A crisp tight line is an indication of whether or not you have a quality minded painter or one more interested in production painting.
There are several key things a painter can do to make crisp lines. When processing your paint project begin with the trim first. When the trim is painted first this allows the trim to be taped off prior to the walls being coated. There are two advantages to this, the roller spray does not texture the trim and the taped trim will have a crisper line.
There is a science to proper taping. The tape must be place directly in the corner and must be straight, not riding up the wall or exposing the trim. Once the tape is correctly placed, press the tape down with a small flat blade (or finger) and apply a sealer to the tape. The sealer acts as a guard against paint bleed through. The preferred sealer to use on the tape depends on the surface you are taping over. If you are taping over painted trim, use the same color trim paint and apply a thin coat of paint directly in the corner on the top of the tape. If the trim is stained, then the best way to seal it is by using a water based polyurethane.
Once the sealer is applied to the trim and allowed to dry, the walls can then be painted. When the wall paint is dry remove the tape and you will be amazed at how beautiful and crisp your new paint lines are.