Speaker Placement First
Before optimizing your walls you need to optimize your speakers first. Check out the Speaker setup section before continuing on with this guide.

Material
The material your wall is made of, plus that of the window treatments, and paintings affect sound very much. Bare walls and floors make the room sound bright and harsh with too much echo. Carpeted floors balance out the room a little, but you'll also need some fabric on the walls to even them off. Using too much fabric and other types of sound absorbers will cause your room will sound dull, lacking in detail and not right. Wallpaper is one of the best ways to smooth out the sound your of room, but can also be the most costly. Finding the right balance usually isn't that hard but requires some time and thought.

The Ceiling
Usually the ceiling is the last thought on anyone's mind, but it is responsible for the worst loss of bass in your system. The corners of your ceiling are bass traps. Some bass gets trapped in the corners and leaves the room never to be heard at all. The only way to fix this is to put a sound ball in the corner. There are a few manufacturers that sell this, but you can also construct your own from wood or foam balls about 4 inches in diameter.

This solution simply makes your corners, round. This round corner reflects the bass back into the room unlike a corner which traps the bass instead.

Canvas Panels
Companies that specialize in room acoustics sell acoustic absorbing panels. These vary in size and can neutralize the sound of your system to perfection. When you purchase these panels many companies offer a guide as to how to position them in your room for the perfect sound. This article won't get into how to position these as placement design is best suited for professionals and home theaters where the goal of the entire room is to be a theater. If you have a stand alone home theater than you may wish to look into these type of products. Otherwise, you can look into making your walls less bare with paintings and the like (which tends to look better).

Window Treatments
These are the most obvious locations you can take advantage of to make your room less vibrant. By using fabric window treatments, such as cellular blinds or drapery, you might be able to balance your room with one fix. Avoid any hard surface window treatments such as metal /plastic blinds, or verticals as they will add to the bright sound characteristics of your walls.

The Floor
Most floors tend to be carpeted, but for those of you that have wood or ceramic flooring, your options are limited. It can help acoustically to get an area rug for your theater. If you want to show of the wood there is nothing you can do to your floor to improve the sound. You'll need to rely more on fabric furniture, and wall solutions to find your room's acoustic balance.

Your Preferences
In the end it comes down to your preferences. The above options are all merely suggestions you may want to look into as you become more finicky about your sound, or notice a harshness you didn't hear before.

As people grow with their home theaters, their tastes in sound quality grow as well. Seeking out ways to tweak the sound of your home theater for improvement is normal process, and part of the fun of owning a home theater.