I leaned massage (including both foot & hand massage) all the way back in high school as a skill to help me get laid, but it has definitely been invaluable as a service to Dommes over the years.

I think learning about the care and feeding of latex & leather goods, as well as basic repairs, are an incredible skill set for a sub to have since wardrobe is so important to Dommes (especially Pros).

Photography / Videography, including how to properly light a space, and post-production (especially the use of editing tools) are incredibly valuable for service to any Professional Domme as well as the ability to do hair & makeup before any shoot (it makes a huge difference & is valuable to all Dommes).

Any online marketing, SEO, social media or other computer/technology skills are great for service to Pro Dommes, but just the ability to do research is great for any Domme (for example, I've been asked to find a good English speaking dentist in Mexico or to find out the process to apply to be a performer at some event over the years even though I know nothing about either of those things).

The ability to cook (for those that cross the dungeon threshold into the real world) has been incredibly valuable to me as a sub, whether it is just to make a simple sandwich or to throw together a full meal for her and her friends has proven invaluable. Earlier this year, I was staying with one of my Dommes & she had to take a family member to the hospital. Well, I knew she hadn't had any dinner and they wouldn't be done until well after midnight so I whipped up something with what she had at her place & brought it to them when I went to pick her up.

Knowing how to properly clean things both from the perspective of what chemicals/products to use on what materials & how to use them (do you apply this with a cloth or a paper towel.. should you use a scrubbing pad, etc...). Same goes for the ability to build things or do small repairs (incredibly valuable both inside the dungeon & out). When I visit one of my Dommes who has one sub who runs a construction company & another who is a professional woodworker, I absolutely did nothing to contribute to that project but at another's place I've replaced a door lock & hung shelves so they are level because it was beyond her.

As for service itself, paying attention to your Dommes needs is the #1 talent/skill a sub can have. Examples: Getting a refill for her when you see her glass is almost empty at a party. Always having a protein bar in case she gets too busy & her blood sugar drops; having her preferential tampons/pads or "Extra-strength Excedrin" (because that specific brand is what helps her) in your bag on the off-chance your visit coincides with her period. Always keeping in mind her dietary restrictions/needs (could be she is Vegan or allergic to cinnamon or even that she just doesn't like onions) so you can grab the right thing from a store; etc...

Another less thought of, but incredibly important skill is being able to leave her the fuck alone when she needs her alone time. I am self sufficient & don't need to be entertained. Again, I remember being & staying in the same hotel room with a couple of Dommes on a particular tour. They were great friends & wanted to just spend time together to bs in the bedroom, so I just went into the living room of the suite & watched pre-season football until they wanted something, then I would immediately get it for them & return to the TV. I've also had many Dommes just need a nap & so I just leave the room & figure out how to waste time while being on call.

I wish I could say I had all these skills but my ability to be given a task & complete it (most of the time) without having the Domme supervise every second us truly invaluable. One Domme wanted me to gold leaf a privacy screen & I literally had never heard of gold leaf, but I figured out how to do it & it turned out great.

Sorry for the lengthy response but I love the question & wanted to give you ideas that night be beyond what you had considered before.