The old-fashioned way of doing this is to have a bottle band about 2/3 of the way up the cylinder. The band is then attached to a diving waistbelt with shoulder straps. Lead weights can be conveniently placed on the belt. If you want to go really lightweight, just use a belay belt, but you will need to find some way of stopping everything sliding around, with the bottle ending up in between your legs, which is none too convenient. What you want is to have the tap under your armpit.
The more modern way is to have two attachment points. You have a bottle band about 1/3 of the way up the cylinder with an integral krab. You then clip this to a loop on your waist belt, Then you have some kind of chest harness with a piece of shock cord at shoulder height which you hook over the tap. In this configuration the bottle sits a bit higher. The main advantage of this confirguration is that you can take the bottle on and off easily which makes kitting up in a restricted space easier.
To make a bottle band, take four large jubilee clips. strighten them out a bit and put them "back to back" in pairs, threaded through one or two mailllon rapides. Place the entire construction over the tank and tighten up the jubliee clips. Now you can thread a belt (or clip a crab) to the maillon. It helps if you put a bit of inner tube on the tank first as you get a better grip and don't shag the paint of the cyclinder....although if you take a cylinder caving the paint is going to get knackered anyway.
Mark