1. Look for diesel pumps. Ideally, you'd use a station that has separate diesel-only kiosks, like CountryMark in Liberty Indiana does.
2. Look for the sticky mess of spilled diesel on the ground, on the handle, and so forth. It's often pretty obvious which pumps have diesel and which don't.
3. Securely tape a note that says "DIESEL ONLY!!!" over the fuel cap if you let someone borrow the car and make sure it has been filled up. Making someone remove the taped note is going to make them stop and think. If you're really worried about yourself, you can do the same thing for yourself.
4. Before you start pumping, take a hard look at the kiosk and your pump, to triple-check that it's diesel. Don't rely on the color of the pump handle.
5. Always fill the car up completely. The fewer times you have to put fuel in, the lower the chance of error.
6. If your car is new, be sure to keep fresh air coming in from outside, so you don't get intoxication from the VOCs off-gassed (the "new car smell"). If you get fatigued from these chemicals, you might be more prone to slip up at the pump. Air the car out before driving it, and try the heated seats to keep you warm if the weather is cold and you have windows cracked.
7. Keep your receipts and photocopy or scan them because the print may fade.