1) set the prop (when you put it on the engine) such that as the engine enters compression, the prop is parallel to the ground.

2) Master Airscrew black nylon props tend to resist breakage better than most others. The Bolly Clubmans are also flexible eugh to resist breakage.

3) Always land into the wind, and slowly pull more and mre up elevator, holdng just off the ground as long as possible.

You'll still break some props... But hopefully not every flight. sometimes, I can get a couple of dozen flights on a prop with my belly landing planes... sometimes, I break a prop every flight of the day, even with landing gear.

(AKA Propbreaker...)

When you put your prop on, and it is parallel to the ground, the engine should be in the center of its free floating range. So, no matter where the prop stops, it will be pushed to horizontal.As fhhuber said on #3, keep it up as long as possible.
Go with MA's also.
I have not broken a prop on belly landings this year, or most of last year. I tend to break them on lawn darting.

I have found that the best way to prevent breaking props, in addition to the good advice offered by fhh and the others, is to always carry a box of spares. It never fails--full box, no broken props. But if I happen to be flying a plane for which I have brought none that day, first landing the darn thing snaps without fail...same rule also applies to glow plugs.