All my pigs came from RSPCA cruelty cases so they have all being terrified wrecks. Just a few pointers I would suggest:
* Talk to them as you walk in the room, you'll sound mad but it works!
* Clean up their cage, move stuff around etc so they get used to you putting your hand in.
* Always approach from the side.
* Subtle movements.
* Put your cage at waist height. I personally don't like piggies cages being on the floor, from experience they are much easier to tame when they are higher up (makes sense, you don't seem as big and scary).
Also, this is probably not revelant to the OPs question but if you have nervous piggies, putting them side by side with confident pigs doesn't half bring them out of their shells!
Eventually you just get into a routine, although they will probably always run away when spooked but when they get to trust you and hear your voice they soon come trotting back.
My pigs are most confident with me as I spend more time with them - I'm always shouting at my partner for not 'telling them' he's coming over and scaring them