The first step should be to understand completely why this is happening. Are there bad estimates? Resources not up to speed? Unmitigated risks?
The two primary corrective tools are crashing (bringing in more resources) or fast tracking (doing more work in parallel). If you are crashing, you need to consider if there is some skill which must be acquired on the job as this may slow down the assimilation of new resources into the project team.
Another option would be to examine the scope and negotiate a reduced scope.
As a last resort, you should consider project restart (starting from the beginning and re-estimating and renegotiating the full plan) or project termination (is this project still viable now that you know its not going to be delivered on time).