In reply to Tall Clare: its quite easy really. when mine was a pup we took him everywhere to get him used to people, different places and most importantly - other dogs. all the while, whenever he behaved in a particular way we liked he got a treat and lots of praise. Now when he goes out he's alaways excited to meet people and dogs. though he's recently started randomly barking at people with hats or hoods which I need to sort out.
The biggest problem with collies is the instinct to give chase to anything not human or canine (though he always wants to play with these), in particular sheep. we were advised to just avoid them as much as possible and work on call back training. this has worked so far but it's a bit impractical when I go running for instance as Its not always safe for me to have him on a lead - he can run A LOT faster than me!
in summary - introduce them to as many dogs, people, experiences as possible. praise and treat ALL good behaviour, ignore the rest though dn't be frightened to use your voice to scare them from doing something they shouldn't, like stealing someone's blueberry muffin at the crag! (true story)