I haven't really finished any maps yet, but i find a good source of inspiration is movies, tv shows, and books. For example, the map i am currently working on was inspired by the warehouse from the end of the raiders of the lost ark.
http://www.youtube.com/watch?v=bg4BbctI7...ed#t=1m44s
http://www.youtube.com/watch?v=bg4BbctI7...ed#t=1m44s