Last year, I used a spreadsheet on the computer. This year, I have the Redleaf Calendar. I'm finding the calendar works better for me because I nearly always have a pen nearby, but I don't always have the computer handy.
I try to do it weekly, although sometimes they sit for 2 weeks. I would rather do it this way, while my purchases are fresh in my mind. If I waited any longer, when I look at that receipt, I would have no idea that gv ptprl3 was actually a deductible roll of paper towels.
I made that mistake last year and lost a lot of deductions. (I didn't know about all of the allowed deductions until the end of the year and had to go back through 10 months of receipts).