On the third zombie map, to get infinite grenades, here are the steps:

1. Hold a grenade in your hand.
2. Blow yourself up.
3. Keep holding the right bumper (grenade button).
4. Have someone revive you.
5. You should still have the grenade indicator on your screen, so when you release the right bumper, you should throw a new grenade.

The timer usually lasts about 3-4 seconds after you throw it.

This cheat can help if you're trying to do the multi-weapons glitch and don't have many grenades.