I hope Artsy does not mind, but I have copied her advice for acheiving crisp paint lines:
Crisp paint edge for stripes, color block, etc.
Hi Everyone! I am a “newby” (my first post) and thought I would start off by offering a great paint technique I use to achieve crisp paint edges. It works great when joining any two colors such as on rounded corners, inside corners, stripes, color blocks, etc. Here goes:
Step One: Tape off the area to be painted using a good quality wide painters tape. Firmly press the edge down (in one direction) on the side of the tape you will be painting into. Don't rub back and forth as this loosens the tape adhesive (I use my fingernail).
Step Two: With the ORIGINAL wall color, lightly brush paint over the tape edge you have pressed down. Let the paint dry. This important step SEALS the edge of the tape. Any paint that may seep underneath will be the original wall color.
Step Three: Paint your taped off area as normal. Make sure that your roller is not too wet when painting into the tape edge. Let the paint dry a bit.
Step Four: Slowly remove tape by carefully pulling straight down. You will be pleased to find an almost razor sharp paint edge.
Note- If your original color is newly painted, let it cure for a few days before doing this technique to avoid the tape lifting off the paint