Something I did in my planescape game to bond everyone together might not be for everyone, cause some players would see it as noseleading but my players liked it. I had that they were all offered an obscene payment for something simple (deliver parchment a to place b kinda thing). Since they were all desolute after being dropped into sigil from other worlds their greed got the best of them. They were leary, of course, so they all inspected the diamonds, which the cloaked figure encouraged. Of course it was a trap. The diamonds were cursed and in order to reverse it the characters had to basically endenture themselves to the man for a time. It didn't last long but it did serve to motivate them to the same goal and when they got free of it they just kept working together out of habit.
As I said, it won't work for every group and is something I recommend only doing rarely in any case.