Measurements

IMO one of the most important things to do before you build, especially if you are stripping parts off another bike (particularly if from your daily rider), is to take measurements. This would be good info to log even if buying a bike off the showroom floor. Park Tool's web site had a faily concise list of these measurements that was a good reference. The link is no longer active so I'll tell you the measurements I took:
1) Seat height: center of bottom bracket to top of seat (along seat tube)
2) Saddle height over bars: subtract the height of ground to top of saddle from the ground to top of grip
3) Saddle to handlebar: front tip of saddle to center of handle bar (at the stem)
4) Saddle angle: lay a clipboard or other flat object on the saddle and use an angle finder to measure the angle
5) Saddle for/aft: Drop a string down from the front tip of the seat alond the side of the bike (weight it with something). Measure haw far the string is before/aft of the center of the bottom bracket.
6) Stem length: center of the bars to center of the securing bolt/center of the upright.
7) Stem angle: using the angle finder again, measure the top angle of the stem. If it's too short to fit the angle finder, you might have to lay a ahorter spacer on the stem to space the finder.

I know this all sound way too complicated but any measurements you can get that can be repeated on the new build will do. These are simple and tend to cover all the bases.
The benefit of doing this is that if you take measurements from a bike you like, you can replicate them during the new build and the bike will fit well on the first ride. Not that you won't tweak it a bit, but you don't want to have all sorts of introduced feedback from your new ride when you are trying to isolate any gremlins from components, feel of the new frame, etc. This is just my opinion but it sure has saved me a lot of frustration. I felt no need to adjust the new bike on the first ride at all! I was already fitted to it so I was able to pay complete attention to how it shifted, braked, climbed, etc.
Addition: I found this info on Park's site. It's weird. You can't find it directly, you have to stumble accross is and link into it.

http://www.parktool.com/repair/readhowto.asp?id=72

There is a PDF file that's a great record!