Like others, I lay it out on design wall & snap a photo.
I have a set of paper plates that I numbered with a Sharpie. I use them in 1 of 2 ways. First, if I have a tricky block, I will use a set just to get that one block pieced together. Then, once I have my blocks made, I'll re-use the plates and put one block on each plate in the order they need to be sewn together. If I have 5 blocks in a row, I will stack the plates together in groups of 5, or sometimes if I don't have room I'll use a paper towel as a row divider.
The paper plates are great because I can easily fit them into large Ziplock bags if I'm taking them with to a sew-in ... and can get them out just as easily.