For the scratches, go a little easier on the screen. I know some games can be really exciting, and the touch-heavy ones (hello EBA) can be pretty detrimental to the screen. However, you don't have to press it as hard as you think you do, the screen is touch sensitive--not pressure sensitive.

The hinge cracking thing is an issue that seems to be popping up in the American DS lites. I've had my DS lite for almost 10 months (got a Japanese one) and it doesn't have any cracks on it. I'm not sure what the deal was with the launch american ones, but perhaps you can open/close the screeen a little slower. Savour the experience, open it in reverence. You are about to partake in gaming greatness (depending on the game). /hippie

For mine, I have a case for it that I use occasionally. If I'm going to keep it in my bag, I'd stick it in the case. I usually have it in my pocket, though.

If you can help it, only use the stylus that came with the DS. I used a retracted mechanical pencil on my original DS when I had Ouendan (which is a little more intense than EBA--the spinners in particular are harder), and the screen never scratched, but it's better to not take the chance.