I like psprof1 and rance's advice on room treatments. I have read that as much as 50% of what you hear is the sound's interaction with the room and the rest is the speakers and electronics. With a dedicated HT room you have more flexibility to install room treatments that absorb, and diffuse the sound bouncing off the walls, floor and ceiling. Low frequency issues can develop based upon the size of the room. Crutchfield Advisor has a nice article on calculating the axial modes of your room. These modes can negatively impact the bass creating boomy sound. Knowing the low frequencies that are a problem can help you design bass traps (often in corners) that can help. Higher frequencies bounce off hard surfaces (walls, floor and ceiling) confusing the listener as same sound arrives at times not intended. There are a number of resources on the internet for making your own acoustic panels that will absorb higher frequencies at reflection points around the room.

It must be pretty exiting to set up a dedicated HT room. Let us know your progress.