I found that the best build log for me was, surprise, the construction manual from Van's. I figured I could either write down what I did on my own, or just use the set of instructions already provided. Writing it down would basically be replicating the instructions that already existed.
As I started each major subassembly, I just copied the pages from the manual (you can print them from DVD now), put them in a 3-ring Builder's Log binder, and as each item was completed, write the date next to it. If I skipped something for some reason, to come back to later, I highlighted it. Non-applicable items (e.g., for quickbuilds) got lined-through. When I was done, I had a complete builder's log of all the items in the manual.
Granted, some stuff wasn't in there (a few specialized or customized items), but those were backed-up by receipts and installation instructions (e.g., exhaust pipes from Vetterman, etc.). I also took a lot pictures and put them on my computer, which was in the hangar for the DAR to look through.
As the end neared, I kept an active "punch list" of items to complete in a spreadsheet, updating it each day, deleting completed tasks and adding any new ones, until one day...there was nothing left to do
Prior to the inspection, I copied all of the preview plans' drawing sheets, and went through them one by one, each drawing and detail, and checked off that they had been completed, installed, etc., to ensure that nothing was missed, with a big check mark next to each and every item. That was more for me and my peace of mind than the DAR, however.
Whatever you use, just keep it complete...as you get further along, its easy to start skipping the end-of-the-day task of keeping notes and taking photos. You'll need the complete set of data to take to FSDO to get your Repairman's Certificate (the guy there was very pleasant, but he actually wanted to see much more detail than the DAR did; in the end, it was no problem to get the certificate).
Good luck, and build on!