Bribery!! Find a food that is irresistible to your pigs. Offer it to them, and hope the youngster copies what the others when they come over for it. Also, as mentioned above, lots and lots of time and patience.
I collected two six week old babies form the Blue Cross at the beginning of September. One (Wenlock) is now pretty trusting and will join the adults in the dash for the edge of their run for tasty tipbits. The second youngster (Wellington) is still very timid although I have managed to get her to take high value food from me when she feels safe sitting under a box in a litter tray; it's very much on her terms though, which is fine, she'll get there eventually.