I buy little moleskin journals from isavirtue here on etsy and use them to keep track of when I sold, what I sold, when I shipped, Where I shipped, how much it weighed and cost to ship, how much I received, discounts used. I can comfortably fit 3 sales per page with that info. I really love her shop, and I love all the more that I can put her products to such good use.
At the end of the year, I use the remainder of the page or a whole new one to state how much I made back for supplies, how much I made profit, how much I sold that year and so on.