Press UP,UP,DOWN,DOWN,LEFT,RIGHT,LEFT ON THE D-PAD. Then press L3 and hold while pressing up and then right. This will restore full health and full time dilation. The code is good for 30 seconds and can be repeated as much as necessary.
If you take and hits while performing the code your life will still continue to decrease. If your life is really low and you don't perform the code fast enough you will still die. Fortunately you will return to the same level, but you will have to start at the beginning.