The flag idea, combined with the taxi tank "ritual", helps for me. I once took off with the vent plug installed before I began using a taxi tank and before using a flag, after having a last minute issue that required a delayed takeoff and a small maintenance action. Saw it on the liftoff - a short pattern and overweight landing made, and a lesson learned the easy way for me thanks to luck. It's that sort of disruption to routine that opens the door. The challenge with using a printed checklist (for me) is multi-layered - first, it's a separate piece of paper that requires a place, and second, the chance of misplacing it in the typical RC outdoor environment (winds blow it away, etc.), and finally the temptation to make an obnoxiously long checklist in the first place that covers the bathroom break, call to the wife, what have you. Maybe a better solution would be to memorize a short mnemonic checklist like glider pilots use in small single pilot cockpits of simple aircraft - RUFSTAL for example for the landing checklist - Radio set and initial call made, Undercarriage down and locked, Flaps set, Speedbrake checked and set... and so on. There is a separate one for takeoff. We really only need one for takeoff but it would not hurt to make a simple one up for landing. That way it stays short and sweet and there is no overly complicated list that is a setup for misuse or confusion.

Just an idea and example thrown out, etc.. I'll just make one up on the fly to see if it could work for some guys - "RUFFFSTAL" - (R) Radio (Proper model selected and battery checked for both TX and RX before flight), (U) Undercarriage (pressurized and swung, brakes charged and checked), (F) Flight Controls (Physical check and function, right throws, rates, direction...), (F) Fuel system (full tank, no bubbles, no PLUGS), (F) Fire awareness (engine start, extinguishers, spotter plan to use the extinguisher), (S) Situation Awareness (Who's flying where, what station box to take, who's your spotter, are the conditions still good to fly, review flight plan / desired actions), (T) Trims and flight modes set for takeoff, (A) Announce runway/takeoff, (L) Look (before entering runway, before turning into anyone in flight, before making a low pass, etc.). As long as you remember the RUFFFSTAL you'll remember your items and you can do the checklist while in motion and with hands full. The "RUFFFSTAL" is not important, just that a short and sweet custom checklist is come up with to hit the big points, and regularly practiced. Maybe just attach a very small piece of paper with your letters on it to the TX and look at it if you get short on brain capacity, and you'll pick up from there.