Here's everything I've tried:
For slip resistance, someone sells these stickers that will make the back of your uke less slippery. They're like the no-slip-strips people put in bathtubs. When I first started I tried one of these and it worked great as long as I was just sitting. Unfortunately, about 75% of my practicing and playing is done walking around, so the solution just wasn't for me.
Next I tried something called a Uke Thong. The problem with ukulele straps that go around your neck , under the uke, and up into the soundhole is that if you let go of the instrument it will fall. Also, the clips can cause damage to the instrument around the soundhole, so you'll want to find a padded one if you go this route.
Someone here sells a strap called a "Mobius Strap". I experimented with a homemade version based on the ridiculously simple concept. It's an okay solution, but I didn't really like it enough to ever buy one. However, it's probably the only strap that will give you hands-free control of your uke without having to alter the instrument.
I like hands-free control. I walk around and play in public when the weather is warm, and last year my Uke Thong just caused more problems than it solved. So, I recently added 2 strap buttons to a couple of my sopranos and bought a cheap guitar strap. So far, I really like them strapped like a guitar, and will probably consider adding buttons to all my ukes if I'm still pleased with this kind of strap at the end of the summer.
