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.
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.
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.