Well, there are a few different ways...
Make sure that all loco and rolling stock wheels are clean. Switch to metal wheels on your rolling stock, as they are less prone to building up gunk.
There are a variety of cleaner cars that can be run through tunnels and other hidden track. We use a combination of two or three at our modular club meets. Don't use anything abrasive though - it'll clean really nice the first time, but the microscopic scratches will encourage the dirt to build up over time. You can also polish the railhead with metal polish.
Don't have hidden track...!
If you want hidden track, make it hidden with a removable backdrop, or on another level. If the track is truly inaccessible, what will you do in the event of a derailment?
Also with DCC, you need many more feeders for power than you might with DC. This will in part help overcome the ocasional section of dirty track.
Andrew