I believe in karma, too. My philosophy is it makes up for the times I get home thinking I bought something, see that I did when I check the receipt and realize I left the shit in the cart or at the counter *shrug*
This is also only if you're walking out and arent already home.
If you're home..... Make a point to do something nice for someone in turn within 24 hours.
Example today... Wal-mart went in for a few things. Got back to my truck and look at the receipt. They didn't charge me for 2 cases of water and laundry detergent a total of 23$. Unloaded my kids and went back in and paid. Now countless times I've gotten home and didn't have all what I bought. All I can say is they are lucky I didn't make it all the way back home.