Simple really - add your best tips for painting (a room, that is). Ideally one tip per answer for ease of voting.
Take off outlet face plates, unscrew light fixtures, etc. Tape over the outlets and switches. Bag the light fixtures.
I've seen a lot of painted-over receptacles. There's no reason to do that, since it's so easy to do it right.
I've seen edged-in face plates and light fixtures. Again, easy to do it right.
The key to getting a good finish is preparation.
Make sure that the surface you are painting is clean and dry, free from any loose flakes of old paint. Fill any cracks with a suitable filler and prime this before applying the colour.
Sand the surface first, then wash and finally brush.
Don't wash brushes out between coats, and don't leave them sitting in thinner. Wrap them in a plastic ziploc bag. If they'll fit in the bag, then great. If not just bunch the bag around the handle and tighten a cable-tie around it.
Keep a damp rag in your pocket for the odd drop here and there. Damp it with the right solvent for the type of paint you are using.
For crisp lines: after taping, paint along the tape with the color that is under the tape and let it dry. Some will leak under the tape (which is ok, because it's the same color) and seal it. Then paint the new color.
Whenever possible, try to work into a wet edge. Don't cut in edges too far ahead before rolling. Painting over a dried area produces overlap stripes!
Use quality materials and professional grade brushes and roller covers. Use the right brush or cover for the job and type of paint you are using. Add in a knowledge of proper techniques, and you will get a good paint job.
Get a good sash brush and practice edging without tape. (For me) it's faster and there's no chance of it bleeding past the tape edge.
Where carpet meets baseboard, run 2" masking tape along the edge of the carpet.
Then use a broad scraper (or something similar) to push the tape down right where the carpet meets the baseboard. This compresses the edge of the carpet, keeping it well out of the way.
When you paint the baseboard, there's no chance of a) getting paint on the carpet, b) getting carpet fluff onto the brush, and c) best of all, then you peel back the tape (careful, do it by pulling back at a shallow angle, not straight up or you can accidentally lift the carpet) the carpet bounces back up totally hiding the bottom edge of the new paint.
I worked as a decorator for many years, and this was one of the trick that took my work to a whole new level of leaving a room looking crisp.
Brush and roller care:
If you're doing a lot of painting, get a brush spinner. The spinner both grips brushes by the handle, and can have roller sleeves slid over it as well. That way, when you wash out a brush, you can spin it dry, then wrap the bristle in kitchen roll. When the brush dries out fully, it's kept its shape rather than splayed out all over the place. You can "cut in" - paint a straight edge - with it straight away next time you use it.
Keep a five-gallon bucket on hand for this, btw! Spin inside the bucket to catch the spray.
You can also do this to clean a roller sleeve. I know that they are cheap, but they shed fluff when you're first using them. For a first class finish, wash the roller before you use it, then spin it dry.
For a short break, wrap brushes and rollers in saran wrap, and squeeze it tight.
Enlist the help of a friend. Pay him or her in beer.
Time spent taping the edges is time saved later. You only have to tape once, and you get the benefit for each coat of paint + each coat of primer.
Since you only have to do each edge once, you can afford to do it very carefully.
Use a power roller. Rolling paint isn't hard for a DIYer but rolling unpowered is tedious compared to using a power roller.
These days, power rollers are not that expensive--less than $60--which is a whole lot less than a full air-compressor/sprayer set up.
Buy a power roller now and you'll save time and also your back.
See this post's answer for how to put paint on evenly to give a nice sheen.
Maybe this hasn't been mentioned because it's too obvious, I don't know.
If you're trying to get a crisp line without tape (maybe between wall and ceiling, semewhere non-critical, or maybe you have steady hands), orient the brush correctly for best results. Instead of orienting to use the width for a wide stripe, turn it 90 degrees so you are painting a narrow stripe. This means all the fibers of the brush are reinforcing the same crisp edge. You'll get a solid edge that you won't need to go over again, thus avoiding further opportunities to drift off the line.