1. Save those Styrofoam trays from packaged meats you buy from the grocery store. They make wonderful paint trays (after you clean them of course!).
2. Some of the best fabrics for miniatures projects are found at fabric stores in the form of "fat quarters" in the quilting section. Just enough for that small scale project!
3. Spackle, or wall patching plaster, makes wonderful plaster or adobe for the exterior of dollhouses.
4. Glue your flooring to styrene sheets, rather than permanently to the floor in your dollhouse, and then just lay the whole thing on the bare floor without gluing it in. This will allow you to slide your flooring out should you need to access the electrical system underneath it someday.
5. Some miniature carpets make wonderful tapestries to hang on a wall!
6. Spray texture (used by contractors) makes wonderful textured ceilings, walls, and even stucco-type finishes! You can find some in the paint section of your local home improvement store.