The closest I have ever come to a gear up on the ground was one time in a Aztec, when I had about 100hrs in it, rushed through my after landing flow and realized to my horror, that instead of retracting the flaps, I had put the gear handle up…yikes! Would have had an unfortunate effect on my career were it not for the air/ground squat switch. Didn't help, of course, that the gear and flap selectors on the Aztec are about 6 inches from one another, and move in exactly the same fashion.
Notwithstanding poor training, other circumstances, like after a go around or missed approach - really, anything that disrupts the normal routine can get you. In the 1980s I used to fly Metroliners. The day was a total loss if I couldn't cross the outer marker at 250kts. which was a lot like taking a freeway off-ramp at 100. You know you were really rushed when after you landed it dawned on you that the before landing checklist never got completed.
So, the two suggestions I have for preventing these kind of mishaps are 1.) Use the landing gear as a method for slowing the aircraft down, and plan your pattern entry so as to have the gear down at the same point all the time…downwind, or whatever. Or, at the same altitude above the field, say 1000ft. Or crossing the final approach fix when IFR. Also, if you ever hear the gear horn, either because the throttle was too far back or the flaps were extended beyond a certain setting - recognize you just made a mistake, don't do it again. 2.)Finally, I see a similarity with gear selectors and firearm safety - inasmuch as it's not good enough to just be pretty sure that a handgun is unloaded…when the magazine is out of it, the slide open and you've checked 3 times, you KNOW it's unloaded. Same with the gear, make dam sure!