I think everyone's pretty much said it all... very important to get a wide angle lens for interior shots. 16-35 is an idea lens, or for crop cameras, 10-22.


As for composition... I have a few thoughts:


I think it's important that your verticals are completely vertical and plumb! Unless you're trying to accentuate a tall space, verticals should be kept vertical in interior photos. It shows that care and thought has been put into the photo.


Also, try and find perspectives of the space that aren't traditional. How often do you see photos taken from one corner of the room and aiming towards the opposite corner? While those are great for showing the overall space, I find that one point perspectives add a distinct classiness to photos, especially if the single point perspective is converging at one of the rules of the thirds points.


Lastly, try and find perspectives from how a person would experience the space. For example, if you're taking a photo of a dining room space, rather than taking an overall view of it, place the camera behind the chair that sits at the head of the table looking directly at the opposite end of the table, and preferably at the direction where you get a glimpse of an adjacent room.


Just some thoughts...