1. Use Frog Tape. The stuff is amazing! Once you've got it on the wall, rub over the soon to be painted edge well to get it adhered as much as possible. (Use your thumb nail, a visa card, or something similar.) If you're walls aren't too textured you can skip Step 2.
2. Paint over the edge of the tape with the wall's original color. Allow to dry. (I usually wait until it's dry to the touch, about 1 hour, instead of the full time listed in the instructions.)
3. Paint the stripe. If a second coat is needed, allow to dry to the touch and do a second coat.
4. As soon as the last coat is applied, peel away the tape while the paint is still wet. Allowing it to dry adds the risk of peeling away the paint you just put on the wall. Not fun!
Does it work?
Trust me - in this room (after painting the stripe twice - due to this mistake) I only touched up three spots that bled - and they were tiny spots.