From Lauren Flanagen on about.com:
Everyone makes decorating mistakes. Even the most seasoned of decorators slip up every now and then. But knowledge is power! Once you know the most common mistakes it's far more likely you won't make them. Here are 15 of the most common decorating mistakes along with a few tips on how to avoid them.
Rugs That Are Too Small
Area rugs should fit under all of the key furniture pieces in a room, with 10"-20" between the edge of the rug and the wall.
The days of matched sets are over! Try to find things that complement each other rather than match.
Too Much Stuff
Editing is one of the most important elements of decorating. Every item needs a little room to breathe, so don't try to cram too many things together.
Not Enough Stuff
An empty room is a boring room. Make sure you have sufficient seating, places to set down drinks, and a few things on the wall for people to look at. And don't forget accessories. They're what give rooms their spirit.
Not Considering the Location of the House
Sometimes what's outside is just as important as what's inside. Tropical décor doesn't look good in a cold climate, just as cozy Aspen style doesn't look great in the Caribbean. Always consider the environment.
One Stop Shopping
Don't buy everything in one store. The items in your home should look like they were collected over time. After all, your possessions should tell the story of your life, not the story of a big box store.
Ignoring Scale and Proportion
Balance is important. While different shapes, sizes and heights should all exist in a room, pieces need to work together. For example, a giant sofa next to dainty side chairs will look unbalanced. Instead, find pieces that speak to each other.
If your style is formal that's fine, but it should never be so formal that people feel like they're stepping into a museum. A home should be welcoming, not intimidating.
Furniture is meant to be used. If your sofa and chairs are not comfortable to sit in, why bother? Art is for gazing upon; sofas and chairs are for sitting.
Purchasing Before Measuring
Always measure before buying. You don't want to get a piece home only to find that it won't fit through the door, or is far too big or small than it should be for the space you intended.
Choosing Paint Colors First
When it comes to decorating a room, paint is one of the smallest investments you'll make. So why choose it first? Choose the more expensive items such as the sofa first, and then decide on a paint color. NOTE: This is a mistake we see clients make all the time. They have already painted, then have to search for the perfect floor covering that works with it. There are thousands upon thousands of paint colors and far fewer flooring colors. Please DON'T select your paint color first!
Hanging Artwork Too High
Artwork should always be hung around eye level. If you have to strain your neck to see it, it's too high.
Keeping Pieces You Don't Like
Sometimes it can be hard to get rid of things, particularly if you've spent a lot of money on them. But if a piece doesn't bring you joy when you look at it or use it why are you keeping it around? If you don't like it, toss it.
Pushing Furniture Against the Walls
Pulling furniture away from the walls help to create cozy conversation areas. If space allows, when you're arranging furniture create seating areas in the middle of the room.
Being a Slave to Trends
Decorating trends are only good when they're easy and/or inexpensive to change. If you're going to follow some trends make sure they're not big commitments. Chances are you'll be sick of them in a year or two