I like to break projects down in to incremental milestones, that way the client sees a portion of completed work, pays for it and you begin the next milestone.

I like to break projects down in to incremental milestones, that way the client sees a portion of completed work, pays for it and you begin the next milestone. This helps keep both of you happy, they are seeing progress and you are seeing payment for work completed. Client management is a difficult thing. I would assure them that the project will be completed, it is taking longer than expected because of x,y and z. That you will do a better job of keeping you informed of issues like these in the future. Give them an updated date that you can deliver early on. If you can break the remaining work into milestones where you can show them incremental progress.