I'm not personally the greatest when it comes to mapping, I mean I really only tried my hand at it briefly because the team I was making games with already had an excellent mapper, lol.
I do know what looks good, but getting it to look that way is another story.
What I would try to do, is look at a helicopter view of whatever it is I'd be attempting to map. Then try to make it look something like that (using that as a guide, basically).
Not sure if that is helpful though. ^^