Use a notebook.
One thing I do before I even start to order parts is to get a notebook. In it I write down ideas, parts, prices, where to buy from, where to find data sheets, etc. Then I would start to draw sketches and measurements. Then after I get my parts I write down what I still have to do and what I still need, but where it helps most is coding for me since its my weak spot. While coding I write down what makes this motor go foward or what is a good distance to turn away from something at or just little notes. This is a great simple tip for beginers (Like myself) and is also good at keeping all your notes in one place about a certain project.