I have been performing Sammy Smith's Supper Frog and have never been comfortable at the moment when the frog is hung on the volunteer's back. I have found that the need to feel secure in letting go has been a problem resulting in a stutter in my actions which draws attention where I do not want it.
After playing around with a variety of holding techniques I ended up taping a small but powerful magnet in one unused envelope in the stack. On the frog I taped a washer about 2/3 the way up (I had to try several to get just the right hold).
The magnet is strong enough to secure the frog. This provides real freedom in handling the envelopes, and allows me to gently rest the stack against the volunteers back, sliding the envelopes away softly. While the magnet has a strong attraction to the washer, it takes very little energy to slide the two apart. So far, this has worked well every time and has really smoothed out a rough spot in my performance of the effect.
I hope someone will find this useful.