Keep them chained or strapped to a wall or post so that they don't fall over. If the Oxygen falls over and shears off the valve, the bottle will become an unguided missile with enough strength to punch through a block wall. (I've actually seen this happen.) Also, ALWAYS keep an Acetylene bottle in an upright position. The Acetylene bottle contains a spongy material that is saturated in ACETONE (which is what actually holds the Acetylene). If the Acetone should get into your regulator (laying down), it will eat up the diaphragm, and Heaven Forbid it should ever get into contact with pure Oxygen in the Torch Head - you don't need a spark to get an EXPLOSION.