Two big questions are the size/properties of the room itself and what sort of system you are installing. If you're just looking for "acceptable" sound and there are enough furnishings, shelving, pictures on the wall etc they you may not need to do anything. If it's a dedicated theatre without much in it, then fabric panels along the walls can make a huge difference. As mentioned there are a bunch of resources for on the internet that deal with precisely this question. One great resource is AVSForum.org, which has
a forum dedicated to home theater design.