If you haven't already, check out this paper written by IKE back in 2009:
http://www.chiefdelphi.com/media/papers/2220
Most of the basics are pretty well covered in there, along with some pretty awesome tricks.
Personally, when I'm designing something, I like to 'plan' how the weight will be distributed. Each subsystem / mechanism will have some amount of weight allotted to it, and that total weight is usually a bit under 120. From there, it's primarily about designing things to be 'light' on the first go around - if it's a part that you know that you only want to make once...
Another thing I like to do is 'hide' weight in places, just in case we need to find a half pound or two later. Sometimes this is done through using (or allotting for) a heavier version of something, an example being the older Thomas compressor rather than the current Viair unit, and sometimes it's through building something out of thicker material than it really needs.