What do you mean when you say, "I always manage to break the the shoulder buttons" - A physical damage? Take it easy on the DS Lite.
A non functional button? Unfortunately DS Lite's are prone to this problem. It's a wide-spread problem. I am sure you must have heard of blow-fix. But, it's only a temp-fix. No pun intended.
If you want to go with a permanent fix, you will have to buy the internal switches and solder them on the board. From my personal experience I want to believe that it's a mechanical failure inside the switch button. Something gets off its way.
You have to be fairly good with surface-mount soldering or have someone experienced do it for you.
Edit: I now see that you have specifically asked for 'prevention'. DS Lite's are really delicate, specially around that region (I meant the shoulder buttons, get your mind out of gutter). So it's mostly just wear and tear. I could only suggest not playing games that are too abusive for the shoulder buttons. For Example: Snaking in Mario Kart DS...