I trained myself a long time ago to keep my wallet out and open while any card I might have given to the cashier is still pending. I take my wallet out of my purse, open the wallet and hand the credit card (or ID at a hotel front desk, rental car agency, etc.) to the cashier. When everything that I need is returned to me, then and only then do I close up my wallet and put it back in my purse. This has saved me at least twice from walking away from the counter without my credit card or driver's license.
When I started MSing, I just added "receipt" to the list of things I need before my wallet gets closed up and put back in my purse. Now if I'm ready to walk away, but I notice that my wallet is still open and in my hand, that means I'm still waiting for something, which reminds me to get a receipt. And when I get it, it goes right into its slot in my wallet so it doesn't get thrown away.
It took some time to train myself to do this, but now it's second nature. I don't know if this method would help anyone else, but feel free to use it if you think it will help.
Shopper in California's Bay Area