Center speaker height, angle, and resonance all listed as common pitfalls.
Audiholics has posted a guide the begins by bemoaning the sad state of common center channel speaker set-ups. In short, with the center channel shouldering a film's dialogue and anchoring the soundtrack and Foley, some common missed adjustments can make a huge difference on home theaters ranging in cost from a few hundred to several thousand dollars.
First off, try to ensure that there is clear air between the center channel and the listener's ear. While this seems simple, the secondary listening positions (for friends and neighbors) is often overlooked and can easily cause volume issues as well as dampening the overall audio enjoyment. This means the height of the center channel in important, but Audiholics also suggest that if necessary, the center speaker should be angled up, which can be accomplished soundly with foam. The fastidious type may even use a laser pointer to verify the right unobstructed vectors for each listening position.
Further suggestions include positioning the speaker close to the edge of whatever it is sitting on to avoid unwanted reflections, using sand in hollow speaker stands to gain a sound solidity, and carpeting anything such as a noise reflecting floor. Audioholics also suggests that the center channel be set to "small" in order to redirect bass to the sub for the majority of full set-ups. And of course, re-calibrating after making adjustments.