You will need pills to do this cheat. You also will need to find a place with deep water.

First find some pills and drown until you are down. Next, have someone revive you and take the pills. Then get out the water and the life you lost while underwater will return to your life bar.

The life from your health bar will increase as well, but will not be on your health bar. Depending on the life you had before you stayed under water will determine your extra life bonus.

If you are playing with bots, they will heal you. This could be good because you could get full life from doing this.

P.S. your life will count down by 1 about every three seconds. Also I'm not sure if it works in Left 4 Dead 2, but it works in the original Left 4 Dead.