Here's another trick: Nothing motivates players to really go for broke quite like knowing their characters are not going to survive the night. Consider the possibility of saying "hey, I want this and the next couple sessions to be a practice run, then I'd like to reboot the game. How about we decide that the characters are going to die in a spectacular fight against impossible odds at the end of session three, and use that as a backdrop for the 'real' campaign that follows? That way we can keep the city and everything we put together, as well as lay down some ideas for what the aftermath is that our next characters will be dealing with."