I once went to a talk given by a psycologist from Farnborough who had been involved in accident investigation. He said he often found that pilots had read the checklist, saying "gear down, three greens' but had not actually done it. They then landed wheels up.
One captain said after landing "there's something wrong with the brakes"
I found that it was prudent to have an extra trigger for lowering the gear that was indpendent of the checklist. just in case.
It's simple. When you see a runway in front of you, check wheels. Erery time. The it becomes a habbit.