Did your adventuring company meet in a tavern? Perhaps they were recruited by a wise old wizard who needed help clearing out a pesky band of goblins? Of course there is always the village in need of rescue and the PCs just happen to be the only able bodied individuals around. Finally, the PCs might be the heirs of old adventurers destined for greater things.
Regardless of the initial start to your campaign your players will require a long term reason for why they continue to adventure together. Often adventuring parties are composed of personalities that have no real reason to remain together. That is where a Mercenary Company can provide the required glue to keep your adventurers together. Wizards of the Coast recently featured two different mercenary companies, Queen Filfaeril’s Blades and The Last Legion (DDI subscription required). Mercenary Companies allow you to keep the party involved with a long term campaign goal, while still providing other secondary adventures.