If you ride long enough, chances are, you or a riding buddy will need to be towed back to the staging area. While I'm sure that there are a few techniques that can used successfully, here's how I recommend that you do it.
1st, NEVER tie the two bikes together. Should something go bad, you need to be able to quickly separate. On the towing bike, attach either to the non drive side foot peg or sub-frame. Not every bike is exactly the same, so there may be other tow points on your particular machine.
On the bike being towed, route the tow strap as close to the center of the bike as possible. Typical good spots are the right side (rider sitting on the bike) of the handlebar mounts (between the handlebar and top clamp) or if possible, between the handlebar mounts below the handlebars, as it's closest to center of the bike. Then, route the end of the tow strap to the left handlebar grip and wrap it a couple of times. The 90 degree angle of the strap will create enough friction to hold tight while being towed, but as soon as you let go of the strap wrapped around the grip, friction is reduced and the strap will pull through, disconnecting the two bikes.
My favorite tow strap is made by MSR and it's both light and easy to carry. I'm sure that there are others worth consideration however. To me, it's just one of those items that you toss in your pack and hope that you never need it. But if you do, you'll be glad you bought one. Or, maybe not if your buddy seems to the one always needing to be towed out.