I must be weird since I never write anything down until I do it. I just keep track of it in my head and that works for me. I do document everything done and write it up as a pictorial essay for future reference though.
Personally, the most important thing anyone can do to get a project completed is this:
Work on your project every day. It doesn't matter how much you get done, just put in some effort every single day and your project will eventually be finished.
I found spreadsheets to be particularly helpful for parts shopping (most of my parts came from overseas). I had a column for each supplier, rows for each part, and prices. In many cases, the most important thing was to find the supplier who had most or all of the stuff I needed to order (minimizing shipping costs).
That particular project is on the road now, but the spreadsheet continues to serve as a record of what I bought and where.