I use a sling system similar to the
one mentioned above, but I just put the hose clamps on top of inner tube segments (with the screw parts in a short length of tube, to prevent catching on things). With my 19cf pony, I put the lower stainless steel hose clamp band right at the bottom of the tank. The upper bolt snap (I used smaller ones than in those instructions) is right at the shoulder of the tank, and the lower bolt snap is right at the bottom. That lets me clip it from my left shoulder ring to my left hip ring on my BC, which is a great spot for it. If I've got a long walk to or from a shore dive, I can also unclip it from my shoulder ring and clip it across from one hip ring to the other, which can make a quarter-mile uphill hike a bit easier (that, and having a bungee to sling my fins over my shoulders).
With it slung shoulder-to-hip, it really does disappear when you're swimming horizontally. Only when you're taking a completely vertically inverted (head straight down, fins straight up) position does it occasionally get in my face, but since I'm not using my hands for much when I'm upside down like that, I just cradle the 19 with an arm. (Okay, if I'm goofing off and log-rolling, I usually hold it, too, but hey.)
I have my pony *only* as a backup in case of catastrophic equipment failure at depth (i.e. blow out an LP line, regulator locks up, etc. -- all things that *shouldn't* ever happen but that *could* happen). When I was deciding how to carry it, the fact it was my lifeline was a *strong* motivation to have it where I could watch it throughout the dive. It's a lot more psychologically comfortable to be able to verify my emergency air supply often during the dive than the very small inconvenience it may pose being slung as I have it (and frankly, once you're used to it, it feels quite odd to *not* have it slung).
I have a full console with SPG and depth guage on my pony (plus a dive watch strapped to the neck as a backup timer). That makes my pony basically a completely redundant system for executing a standard ascent in the event of a primary system failure (lost air, computer died, etc.). Additionally, if my primary SPG (HP hose, whatever) goes south (or my buddy's does), I can pull the console from my pony and swap it out for the bad one. (I even have a button SPG in a padded film canister so I can still carry the pony.) A week and a half ago, I actually had to pull the pony's console to save my buddy's dive. Worked precisely as planned.
(Incidentally, I saw a liveaboard's site one time that actually used the same pony rule I use. If you run out of air, your diving day is over. If you use your pony, you have run out of air, and your diving day is over. If you are forced to go to your pony
due to equipment failure, and you can correct the problem, your diving day can continue.)
