Take what I'm going say with a grain of salt, but it has worked thus far with me with both current pigs. I just got a young female yesterday (took to vet today, weighs 0.8 pounds) and guessing she's around the 4 week mark. She's pretty timid, but nonetheless was skittish. Just today, she took veggies from hand (within 10 minutes of returning from the vet, so just to put it into perspective), a couple times and has been letting me pet her in the cage.
A trick that'll sound completely ridiculous I started with my boar and carried over to my female, is pretend my hand is another rodent. Basically, thumb, index finger, pinky, and ring finger "walk" around like a small animal with legs, while my middle finger acts like the head (think of an aardvark or elephant). I'll add fresh pellets and timmy hay, then immediately "walk" around on the fingers, while "poking" my middle finger's tip into the pellets to make noise against the dish. Then, I'll "walk" over the hay lump, and poke that, acting like it's "eating" or "foraging". So far, it works about 98% of the time assuming they aren't tired. The female especially, stops the skittish behavior, and will come over (remember, just got her yesterday and roughly 4 weeks; the boar is about 2 weeks older and does the same thing to a certain degree). If they come to "sniff" me, I'll raise the tip of my middle finger, and get close to their nostrils for a very brief second (lets them smell the food you were poking and mimics pig to pig behavior) then immediately BACK to the food/hay. If they run away at other times, I'll pretend to run away too, in opposite corner (again, mimic the legs running and kick up bedding material) then go right back to the food. With the male, I'll sometimes "follow" him at same speed he's moving, bump his rump, pretend to "popcorn", then run away. The female, I'll just popcorn around and run away if she runs, then go back over to her.
So far, this seemingly ridiculous "trick" has worked with excellent results. They seem to ignore my arm or "see" it as an extension of the weird little rodent thing walking around their cage.The female will actually walk directly under my arm and "behind" the hand rodent to sniff. If calm, you can walk over, and slowly pet them around the shoulder blades and talk to them. Then slowly move hand away and go back to "hand rodent". Also, IF they can't see my hand, I'll rub my thumb against my index and middle finger. That "sound" indicates I'm there, and not trying to sneak up.
So "hand rodent walk", pretend to "eat" by poking food/hay, pretend sniff, back to food, run away and "popcorn " (just hop hand a couple inches and scurry to opposite end). If something has a texture, I might scrap my middle finger nail on it, to simulate chewing, etc. Unsure how all of this might work with older adults, but both younger pigs have responded favorably. Just keep hand low to the bedding and finger tips on/in the bedding and move middle finger like it's sniffing the ground (or looking for ants

) and maybe articulate from side to side like a turtle neck. Seems to completely disarm them. So instead of threat, you "appear" as a weird stick rodent with no eyes. Also, if you try this and get to the stage where it lets you pet them on the shoulder blades w/o running away, that can sometimes be a good time to pick them up (slowly- don't want to act like a predator getting a guinea pig dinner). Though try to alternate it up, so they don't associate the petting with instant pick up i.e. walk away at times after petting, etc. GL.