Low tech version is have a giant message written into the carseat to remind you.
That’s obviously not effective, or you would have recommended that to remind you not to leave your child in the car.
So what I used do was, I used to always walk around with only my left shoe on. This way, when I would drive, I would have to put my right one on, (see http://www.theyeshivaworld.com/coffeeroom/topic/low-tech-tip-to-help-prevent-child-left-in-car#post-612700), which reminded to take my left shoe off.
The app is more comfortable.