So, it seems the basics are out of the way of both keeping the party together, and getting players to work together. I think @
iserith and @
Oofta covered most of this pretty well. But once you all agree to work together, and your players have created their characters... how do you get them on the road to adventure?
I find that the easiest way to get them to work together, is to drop them into an exciting situation right away. If they have to work together immediately, then they are less likely to go at it alone later on.
Then there are the stakes. So what if the evil dragon is threatening the nearby kingdom? Why should they risk their necks? Part of this can already be tackled in session 0. You could simply agree with your players that the premise of the campaign is all about defeating the dragon, and so with that established, it is now up to them to direct their characters in that direction.
But another way to solve this issue, is to give their PC's a personal stake in the matter. In some cases these stakes can already be established during character creation (The human rogue has a debt to settle, so he needs the job. The paladin has sworn an oath to defeat the dragon, etc), and other times you can let these stakes emerge during the campaign. A dear friend (and npc) of the party could ask them for help.
I did all of this in my current pirate campaign:
Session 0
-I first agreed with my players that they would form a band of pirates together in an aquatic/naval campaign, which would be all about exploration, adventure, and piracy.
-I agreed that although they were pirates, they were basically good guys, and not evil (well, they're alright, but they are not murderous thugs).
-I agreed with them that they would work together, and not attack each other, or steal from each other.
-I agreed with them that the premise was that they would get a ship, gather crew, gather allies, and eventually face a huge enemy fleet in a climactic end battle. The rest was up to them.
-During character creation, I gave them some backstory options that they could use. I also asked them to think about whether any of their characters perhaps were already related to one another.
During play
-I dropped them into an action scenario. Half of the party was locked in a horrid prison tower, while the other half was climbing said tower in an effort to free them. As they escaped the tower, they made some allies and enemies along the way. Having escaped from the same prison gave their party a foundation on which to build further adventures.
-I made them care about the world and the people in it.
-I lured them into various (side)-quests with treasure and adventure.