The biggest threat to a good paint job? The clock.

From choosing a great color that somehow turns hideously wrong to completing a paint job that looks gorgeous and then starts peeling, most painting mistakes trace back to a lack of prep work.

If you want to make sure your next paint job looks like something out of a home decorating show, here are the biggest mistakes pros see -- and how you can prevent them:

1. Jumping the gun. Deciding you've got to buy the paint and paint this weekend. "Do not go shopping for paint intending to bring home paint that day," says Christopher Lowell, the Emmy-winning lifestyle expert and author of "Seven Layers of Design." Instead, he says "grab swatches -- as many as you like." Put them in a high-traffic area of your house, and see which ones you gravitate toward, he says.

2. Dreaming up colors. Pulling your color selections out of thin air. "The best way to choose a color for your space is to start with an inspiration that is visual," says Maxwell Gillingham-Ryan, author of "Apartment Therapy" and a frequent makeover expert on HGTV's "Small Space, Big Style." It can be a rug or a photo from a magazine. In either case, you aren't left trying to describe your idea of a color. You can simply point to it. Best of all, since your example has already been created by professionals well-versed in color, it's an almost foolproof way to come up with other complementary colors to use in your room design.

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3. Pursuing trends. Never mind what the "hot" colors are this year. You're living with your choices. Choose hues that make you happy. One good clue for you is to consider what solid colors you wear most often -- besides black and white. "If it feels good on you, chances are it will feel good around you in the home," says Lowell.

4. Mistaking swatch for wall. Getting the "right" version of a color can be tricky. Colors will look darker on the wall, but the glossier the paint, the lighter it will look. First, use swatches to narrow your choices. Once you have a handful, get small containers of the paint, and test the finalists on big pieces of poster board, says Sharon Hanby-Robie, designer and author of "Decorating Without Fear." Prop the boards up in the room you'll be painting. Look them over the next few days in all kinds of light conditions, from daylight to nighttime illumination. "That's the fun thing about color -- it changes with the light," she says. Shades can also take on aspects of the other colors near them, like carpet color, which is why you want to test it out in the room before you paint. After a couple of days, you should have a favorite. And if you don't, at least you don't have to live with a bad choice until you repaint.

5. Ignoring warm and cool. Not acknowledging that color can change the perception of a room. Warm colors "are expansive," says Gillingham-Ryan. Use them for social areas, like a great room or dining room. Cool colors are calming. They can be great for a home office or bedroom.

6. Forgetting every wall is different. Want to give a room punch by painting one accent wall? Select an unbroken wall (no doors or windows). Often it's the wall you see first coming into the room, says Gillingham-Ryan.

7. Forgetting the ceiling. "It's the fifth wall," says Hanby-Robie. But even experts disagree on how it should be painted. If you expect to paint your ceiling a color, consider going a few shades lighter because ceilings tend to look darker. Some designers prefer a white ceiling. A decorator's white (not a stark white) will give you a sharp definitive line from painted walls and "highlight whatever architecture you have," says Gillingham-Ryan. Others want the ceiling in the same color family as the walls. "If you paint the walls a rich color and leave the ceiling white, it will do the opposite of what you want" by calling attention to it, says Lowell. Instead, he prefers a variation on the wall color. If your ceilings are above nine feet, go one shade darker than the wall color, he says. That will bring the ceiling "down" and make the room appear cozier. If the ceilings are nine feet or less, use a color a shade or two lighter than the walls.

8. Not using color to connect rooms. Especially those that flow together. One shortcut to getting a pro look: Take a swatch that you like with seven versions of the same color. Eliminate the darkest color (you can use that to accessorize the room later), says Lowell. Now go two shades lighter. "That's a great wall color," he says. Try two shades lighter than that for the ceiling. And use the lightest shade on the card for your trim. Want to coordinate two adjoining rooms? Use the same trim color, but switch the wall and ceiling colors, he says.

9. Not identifying existing paint. Many older homes have oil-based paint on the walls. But homeowners most frequently turn to latex varieties when they want to paint. The problem: You can't put latex over oil unless you've coated it with a special primer first, says Hanby-Robie. Otherwise, your gorgeous new paint job will peel off within the week.

10. Thinking all paint is the same. You have latex and oil. You have a variety of paint finishes from flat to eggshell (matte with just a hint of gloss), to semi-gloss and high-gloss. For a bedroom, you might choose flat or eggshell, while the adjoining bathroom gets the same paint shade in a more water resistant semi- or high-gloss.

While some of today's popular darker shades look too "chalky" in a matte finish, an eggshell has just enough gloss in it to give a professional matte appearance. Flat paint hides imperfections in a wall the best, while glossier versions can highlight them. "Glossy paint in the hands of an amateur is a no-no," says Lowell. His recommendation to do-it-yourselfers for trim: eggshell.

But Gillingham-Ryan believes that homeowners should choose trim finishes based on the style they prefer. Glossy trim will give you a more traditional look, while eggshell trim imparts a more contemporary appearance, he says. If your home is new and the walls are in good shape, a glossy or semi-gloss accent wall can look great.

In addition, there are special easy-to-clean versions that resist stains and scratches and eco-friendly paints low in volatile organic compounds, called low-VOC paints, which emit much less in the way of smells and gases.