-Note: This trick may no longer work after the latest game update. Pause the game on Xbox Live while low on health, then rotate the D-pad in a full circle at least three times. Choose the "Achievements" option at the menu, then press B to exit that screen. Select the "Resume Game" option. You will remain at the same screen. Select "Resume Game" again and your character will have full health.
-You can also pause the game on Xbox Live while low on health. Press the Xbox Logo to enter the Dashboard menu. Press Xbox Logo again to exit the menu, then resume the game.