From “The Mythical Man-Month” by Frederick P. Brooks, Jr.:

„The bearing of a child takes nine months, no matter how many women are assigned.“

 
This picture shall describe the fact that adding additional man power to a task doesn’t affect the schedule. This is true for tasks that cannot be partitioned.

 
In fact, I have seen projects were additional men were thrown in and the schedule got even worse. One reason for this is that the quality of communication goes down. But good communication is the single most important factor for the success of a software project.

 
If you sense problems with your schedule the first thing to do should be to find where that comes from. Consciously watch out for ways to improve communication. Always favor face-to-face communication over using a bug tracking system, instant messaging or email. See if there are other obstacles that get in the way of communication. Getting rid of unnecessary office walls or installing an additional whiteboard can have a huge impact.

 
Other ways to improve the quality of a team is to train the members, providing better tools and improving the development process.