We have a sort of unwritten rule that each PC knows at least one other member of the party somehow and this is usually enough to provide group cohesion in the early days before we've all saved each other's lives a dozen times. We also have a much firmer rule that you may not make a character who "does not play well with others". If you want to make a "brooding loner", that's fine. But you'd better bring along a second character because we're going to be leaving the "brooding loner" alone and brooding. Given that, we don't have a lot of problems getting the group to come together and stay together.
As for specific ways to start the campaigns, one of my favorite ideas comes from a one-shot adventure I ran a couple of years ago:
Each member of the party gets an invite to come to a meeting place (I used a tavern but an outdoor location works pretty well too). They get there and meet each other when the person who invited them shows up. He (or she) says "May I have your attention?" Throws back his cowl and voila: He's a medusa and they all turn to stone.
After an indeterminate amount of time (whatever you like), they are awakened in the dead of night and they are wet. They've been awoken by a mysterious benefactor and the following points are made clear:
- The Medusa has taken over the whole province in the intervening years.
- He took the PC's out of action because they were just the sorts of upstarts who would have opposed his rise to power.
- The "statues" of the PC's were left in place as a warning of what happens to any who oppose the will of the Stone Lord.
- The magic potion they've just been annointed with has turned them back to flesh for a limited time (maybe a week, month, year - whatever seems appropriate for the campaign). The only way to make the transformation permenant is to mix the blood of the Medusa with some more of the potion and drink it.
They are all well motivated to seek out this goal and the Medusa (once he notices that his favorite statues are missing) is well motivated to track them down and kill them giving you plenty of potential encounters for their first adventure. Once they've managed to slay the Medusa and restore themselves permenantly to flesh, they'll likely have spent enough time togther to have bonded somewhat.