You probably already have a spreadsheet to help you track submissions -- you know, which stories you've submitted, how long they've been under consideration, and so forth. That's great.
I'm here to tell you that you ALSO need to keep a spreadsheet for your sales, including the date you received the contract, whether you've been paid, the amount due, when the exclusivity expires, and so forth. Do it now, before you have a big inventory. Because once you've sold twenty-odd stories and someone asks you for reprint rights, you really, REALLY don't want to have to dig through your email looking for the original contract.