A Poorly executed paint job is easy to spot, with its drips and slips and uneven lines. More often than not, haste is the culprit. But with some thorough preparation up front, many of the possible pitfalls will be eliminated—and the actual painting will be much easier, as well. So before you crack your first paint can, schedule in a full day for prep.
The best prep starts with sanding and sometimes requires scraping. Sanding scuffs up the existing surface so the paint grabs hold; scraping will take off any old bumps or drips. (Be aware of how many layers down there may be lead paint, last sold in the early 1980s; sanding isn't likely to reveal it, but scraping might.) You should also take your time patching holes, caulking gaps, and cleaning dust or grease with soap and water so you start with a pristine surface.
If you're working with new walls, or if you've patched any holes before you start, you'll also need to prime. Primer fills in spongelike pores and creates an even, solid base that takes paint well. You should also prime if you're going from a dark to a light color or vice versa. (In the latter case, have the paint store tint the primer to go with your wall color. It will save you at least one coat of paint later on.) Primer should also be sanded before the paint goes on the wall; for a top-notch job, sand between paint coats, too.
First-time painters tend to tape every edge to guard against misstrokes. But that can bring on its own problems if paint bleeds under the tape or if you peel paint off when removing it. It's more efficient to use angled brushes, which draw a straight line when turned on edge. Proper use of an angled brush can save you hours of setup and touch-up.
Angled brushes also come in handy for cutting in along corners and ceilings and around trim, where rollers can mark. Fill in a few inches at these spots, then marry the line with the field by rolling over them. Keep a wet edge to avoid dark spots or paint pulls.
As you paint, be sure to keep moving: Put the paint on the wall, level it out where it's heavy, and get on with it. Don't use back-and-forth brushstrokes, and don't spend time making the first coat look perfect (it won't be). Finally, take a tip from the pros and always keep a damp rag in your pocket to quickly wipe up your mistakes. Even the best painters color outside the lines once in a while.