I don't do much interior work, mostly exteriors, but here are my thoughts.
camera setup - Use the 2-point perspective camera to make all of the vertical lines vertical. Spend a lot of time perfecting your camera setup using all of the tools available to you (field of view, aspect ratio, etc.)
high detail - with interiors, detail is REALLY important. make sure you're using fairly high polygon models (you can't go crazy but SU can actually handle more polygons than most people think). Use Fredo's RoundCorner plugin (http://forums.sketchucation.com/viewtopic.php?t=10370) to bevel all of your hard edges. This makes a huge difference and really separates novice from professional work.
materials - I don't know how much control over materials you have in IDX but with simple materials you'll always end up with fairly uninteresting results. To get an image to 'pop' you really need variation in bump, reflection, etc. Also small things like subtle dirt in corners and slight color variation even in things like painted walls will add realism. As was mentioned earlier, don't be afraid to use photoshop to help with some of things things.
-Brodie
