I have tried a number of systems. I don't use a sling because I hunt, including crabbing which means I'm almost dragging on the floor as it is as I "wrestle" with those dungeness.
I will use a sling if I'm just "touristing" around somewhere, however.
I've got a couple of systems that work for attaching my 13 and 19 to my main tank. The one friend above is correct, one band is enough for a 13, but I prefer 2. So this is what I have done:
1. I have 2 straps around my main tank attaching it to my BCD. My BCD only comes with one strap, but it has an option for two, so I did it. Some BCD systems are harder to add the second strap.
2. I take a length (enough to at least go around twice) of double sided 1 1/2 inch velcro strap and wrap it around the two staps. This forms an "anchor" in which to attach the pony to in a second place.
3. I use the attachment that comes with the attachment kit and then I simply get about 3 (or so) feet of double sided velcro band and wrap once around the pony and then another time around the "anchor" strap and then around the pony again.
You can buy the main attach system I'm going to describe to you, but I found that I could make it just the way I wanted. I got two tank straps and attached them together with a generous length of that 2 sided velcro strap stuff. One of the bands goes around my pony and the other goes around the main tank. Switching out the main tank is very quick as I simply pull the buckle open, as I do the others around the main tank - then the main tank slips out. Slip the new tank in and complete your buckle closing (and reattach the reg) and you're done. Nothing to it!
By the way, I always leave the air on to the pony attached to the main tank, and I have a pressure gauge on a hose that I can read at all times. Also, I decided that 13 is fine for shallow dives (less than 50 ft), but deeper than that I want the 19 cf. So for ease of use I have just come to use the 19 cf all the time. Also, be sure to remember that if you ever get in an out of air situation = the pony does supply air to your BCD, so you'd better practice oral inflation - don't wait for an emergency - practice now.
This system is so easy to use. One nice thing is that you don't have to put any steel strap around the pony, so it really is a portable/removable system. I take it with me when I travel and get the pony overseas. But I have to admit that last time, I was told a pony would be available, but there wasn't one to be found to be rented. The tech guys all owned their own, but none to be rented. No big deal, I could sling, but I like my system better. Next time I'll think about taking my pony in my check-on luggage, but it must be open so the airport security can inspect it.