It does take time, but the more calm and slowly you take it, the more they will trust you. I agree with beaney, try and be around them as much as possible but without trying to pick them up or stroke them yet. Lots of chatting to them them too.
Grass seems to be a favourite. I would suggest that as a starter for hand feeding. When you offer the food, you could hold it just out of their reach, so they have to take a step towards you. It's just a little thing, but eventually they will come all the way across the cage to your hand & feel safe being close to you and it's their choice rather than them being cornered etc. (that's if they're not already brave piggies and come up to your hand!)