Well, here I was all set to do you up a nice article on creating good soundtracks and Mechspike from Yog-Sothoth.com bet me to it. Don't take my word for it. Why not take a look at his fantastic write-up and all the helpful thread comments to learn more? And while you're there, you might as well take a look at the various other parts of this wonderful web-site.
Don't wait ... venture over to this thread link today.
(If I'm going to direct people to a link, I might as well be all canned commercial about it.)