Wedge has something written at
www.map-review.com) about making levels look nice ;)
But yeah, like Codja said, ill take like 20-30 screenshots of levels in JA and JK2 for a reference in making maps. :) Ill even pause my star wars videos at certain scenes just to attempt to get my map to look right. Remember, never leave walls flat, make curves, or right angles... stuff like that. Add support beams. For the ceiling add lights sticking out (or going inward) for the floor, you could make very small cuts and texture it differently. If its terrian, try and blend the textures.