I know a lot of people get stuck on this so I thought I should make a thread. I'd say try to get as many Trophies as you can before hand so you can access the weapon select at any time. Also use super slow motion from time to time, helps especially on the laser target guys or shotgun dudes that storm you.

You can't weapon select during Crushing, it's disabled.

Tips: Equip the shotgun and shoot from the hip. Let the enemies come to you.

DO NOT USE the cover system. The game is a million times easier without it. the trick is to use the environment for cover but dont actually use the system i.e. dont press circle to press up against it. Instead just sit behind a wall or whatever and let them come to you and if they are not then aim your gun and slowly strafe to the right or left and you can get a clear shot on most enemies without them being able to shoot you. The enemy AI seems focused on the assumption that you use the cover system so playing it this way gives you an advantage. once you condition yourself into this style of play crushing mode  is a piece of cake

So I just started my Hard run before Crushing, do I still get the trophy on hard/crushing if I use the rewards?

"You can't weapon select during Crushing, it's disabled.

Tips: Equip the shotgun and shoot from the hip. Let the enemies come to you."

For me the most important thing was to learn where the aim would be once I was out of cover. If you know that you can take out an enemy without wasting time to aim, and thus, without being shot.

ALL HELPFUL REWARDS ARE DISABLED!

As somebody said above, don't always rely on the cover system. There are a few places where you're better off not using it.

You need to get really good at getting head shots. That's key. Also, use your automatic weapons effectively by aiming at about their chest and allowing the kick back to make a line up to his head. Also, only use the shotgun when the enemies are close enough to fire from the hip. Finally, the sniper is your friend! Use it whenever you get the opportunity.

Make sure to join the Platinum Club! (look through the Uncharted forums)

I got to the very end of hard mode before I had to return it. Wasn't difficult, but of course it wasn't crushing either. I'll rent it again some other time, that way I have a full week to beat it.