My Rohloff Speedhub is pretty slick, but unfortunately so is the twist shifter when I'm sweaty. In southern Florida, riding my trike, that's pretty much all the time. Switching gears already has higher resistance than your standard derailleur; when under a bit of load it takes even more effort, and there are times when I literally cannot shift gears because my grip slips. I don't have many options to change out the shifter (it's custom to Rohloff) and it's irregularly shaped so I don't think I could use bike tape or anything on it. I'm hoping someone can give me a solution that will give me a good grip even when sweaty or wet that isn't going to look ridiculous or be unwieldy.
Cycling gloves are designed for comfort and to improve grip by absorbing sweat and providing a contact material that provides grip even when damp or wet. Typically, hands do not sweat much so most cycling gloves will help in this regards.
A thumb /trigger shifter has been developed for the Rohloff speed hub. It has been manufactured by Cinq5 and is called Shift:R. It involves two levers one for up-shifting and one for down-shifting. I assume it will take some ride time to get used to the new and some what unusual methodology. They are not inexpensive. Reviews have pros and cons so you would want to read a few and decide for yourself, as product recommendations are off topic here.
Gloves ought to do the trick, but if want a solution where you don't wear gloves, you could use sugru, the "moldable glue" to make the shifter more ergonomic: https://sugru.com/about
How worn is your shifter? When new they should be grippy and tactile, but with age, sweat, ozone and UV the rubbery grip will break down just like the hoods on a brifter.
I've successfully used electrical heatshrink to cover cheap MTB grips where the rubber has gone tacky or otherwise failing. Perhaps you can fit a short piece over your shifter then heat it with a hairdryer to tighten onto the shifter neatly.
It should simply fall off once cut, if you don't like it.
In the same vein, you could use a piece of bartape or adhesive hockey tape, but those have a sticky back and will be harder to remove cleanly.
Minor benefit, increasing the effective diameter of the shifter will make shifting feel easier because of increased leverage.
I have the same issue and am going to try a remedy I came across online. Wrap a couple of those big, fast broccoli rubber bands around the shifter. Tape of any kind is a bad idea -- the adhesive sticks won't come off and trust me, do NOT use goo-gone to try to remove it. It'll melt the grip. I got mine reversed with quick action and it dried/hardened again, but I thought I'd ruined it.