Weirdly, I've been thinking about this too. I ran naked for a few days and really liked it however it is insanely slippery!
I started searching for mobile "grips" as I wanted something I could "apply" to certain areas. For me its namely bottom right corner, bottom left corner as that's where contact is when I'm holding it one handed. However all I could really find either looked horrible or poor DIY jobs.
I wanted something transparent, low profile but has a slightly "grippy" texture.
I'm thinking about checking out DIY store for adhesive tapes etc. and seeing if anything is suitable. Other avenues might be worth exploring, those sticky back gel things used to apply to ornaments etc. to prevent scratching surfaces they're placed on. And the sticky back materials they use on the bottom of chair/table legs to prevent scratching wood flooring.
For me, even a small square of material which is super thin, transparent and otherwise unnoticeable on the rear bottom half of the handset would suffice for a decent grip.
What would be even better though, is a low profile bumper which lips around front and back adding grip around the edges.