This is such a simple, but savvy tip. Instead of paying your friend back in person, use PayPal to pay her back. Reader enginerdette commented on my post about different methods of online payments, and she says she uses "PayPal more for personal transactions (like paying back friends... it beats giving them cash or check.)"
There are plenty of benefits of using PayPal to pay your friend back. For example, you'll be able to do it immediately, and not have to wait until the next time you see your pal. Since you're able to pay her back instantly, there's less chance of you forgetting to pass along the cash to your friend. Another benefit to using PayPal is that you'll have a receipt that's saved in your account and sent to your email, so you won't need any paper receipts that you can easily misplace or lose.