Back up gas. Not an octo or air source type thing buta completely redundant gas source, pony bottle or independant doubles. Leave the spare air in the store. Back up depth and timing devices. Back up computer(your brain with tables or wheel). Spare lift bag or smb deployable from depth, spare reel or finger spool. Spare mask. Detailed dive plan left with someone on shore. Complete confidence in your gear (read regularly serviced), confidence in your skills. Including self rescue skills. Way above average buoyancy skills and control. Knowing when to call a dive. Experience and more experience. Training above and beyond BOW whether formal or with very experienced buddies. Think rescue diver or higher. Experience in low vis, currents, cold water, etc. If you have all of this and a specific reason for doing it such as one of those "two may be one but one of the two is a trainwreck buddy"go for it. This is the place to come for opinions on diving solo and what you may need to do so. IT IS NOT A SUBSTITUTE FOR ALL OF THE ABOVE REQUIREMENTS AND TRAINING! We are not here to instruct you just to offer tips and techniques and while this is not a condemnation of the practice and not to scare you away from it you need to understand the risks involved. At the same time there are those who are absolutely against it and they have a right to those thoughts. But this is a forum for solo divers and those interested in the activity. If you are so against it I respectfully ask that you realize that this forum is for those who are not and find it a safe, fun, and rewarding activity.