I've had great training and good habits, too. But after 50 years of flying, I landed gear up the first time. It was a long (50+ miles) final glide into Harris Hill at the nationals a few years ago, below glide path nearly the entire way. To my knowledge, no one had been ridge soaring that task but I thought the wind favored it as I approached across the valley. I connected with the Hill just below the crest and turned left to run across it. I gained enough on one short pass that when the finish line/taxiway came into view out my right wing, I simply turned right on very short final to land/finish. I appeared low from behind the trees and turned in so suddenly that no one had a chance to radio a warning. My primary concern was that I was flying downwind and needed to get it down so I didn't roll off the back of the Hill, and also avoiding the crop of finishers landing (properly) in the opposite direction.
I never shifted into "landing" mode. Call it tunnel vision: I was focused completely on finishing until the last few seconds and didn't even think about my landing checklist. There's an emergency field below the Hill that was available if the ridge hadn't worked; I deduced later that probably gave me one less reason to think about landing vs. finishing.
I'd had a gear warning horn in another glider but landed one day in the early 70s with it going off in my ear anyway (the wheel was down but the lever wasn't locked all the way over against the cockpit wall, which didn't cause a problem). I always figured: why have a system if I was going to subconsciously ignore it anyway?
Since then I've used a clip that I move from the gear handle to the dive brake handle when I release, and back again after the gear goes down. I could probably ignore this one, too, but it would be more difficult. That doesn't mean I couldn't make the same mistake again and I'll take all the help I can get. I called "gear down" in the pattern for a while after that. I may start doing it again. Or install the microswitches and connect it to my ClearNav vario this winter.
I was very depressed the night after I bellied in. I was gratified but amazed by the number of experienced pilots who dropped by the hangar to reassure me by confessing they had done it, too, some of them twice, one at an airshow in front of spectators. If the saying about "there are those who have and those who will" applies, there are a lot fewer of "those who will" remaining.
I don't scoff at anything pilots do to avoid making mistakes. None of us is perfect.
Chip Bearden