The best way to stay alive is to crouch down, but do not lay down. Find good cover, and any chance you have, get a machine gun or a semi machine gun. When you are crouched down, remain that way. It makes you a smaller target and it does not affect how fast you run. Also, there are a good amount of health packs, but you still have to look for them. If there is a room that you do not have to go into, you should still look in it regardless.