Easy Energy
In order to gain more hit points (up to 5, for a total of 8), collect as many cartridges from defeated enemies as you can. An easy way to collect the cartridges is to find a communications room that has an enemy soldier nearby. Kill the soldier, take the cartridge, then enter the room. Leave the room, and the soldier will have returned! You can do this as many times as you'd like to stock up on hit points.