I have one of these as well, although I'm not sure why as I really don't like double-sided blades.
A paracord lanyard helps with the grip, if knotted at the handle.
To make it less slippery you could wrap the handle with either paracord or hockey-tape.
Hockey-tape could only be wrapped at the back-end of the handle or it would interfere with the detent of the sheath.
Paracord would be difficult unless you added another hole in the handle near the blade. I think it would also render the sheath unuseable...
Another possibility is to add handle scales - again would require another hole drilled in the handle & would make the sheath unuseable.
Perhaps a kydex sheath could resolve some of the issues...
The Sting is almost ideal size for EDC, but the double sided blade (for me at least) makes it pretty impractical for most things - unless you want it solely for self-defence.
I wonder if it could be reprofiled as a single side blade????