I didn't have a case on my original iPhone and I don't have one on my 4. I got a speck case for it, but after a few weeks I missed the feel of my phone. I love these phones not only for how they work and look, but how they FEEL. I've had my 4 fir around three months and not a scratch. By the time I got rid of my original iPhone, sure it had some scratches, but nothing bad.
My phone is always in a empty pocket or sitting on my desk. That's it. I think if you can honestly tell yourself that you can take care of it, you don't forget your phone when you throw your crap on the floor, leave your iPhone naked.
I always have mine in an empty pocket. The right one is just for phone. I am always conscious of where it's at. I don't have a lot of money it was an investment for me and I won't be getting a new one till the 2 years is up. I have scratches still. No idea where they are from but I WAS careful. I don't know how you do it.