I was asked the other day “If a project is behind schedule,  what can you do to get it back on schedule”.  Assuming that your estimates were accurate and your team efficiency can’t be improved,  the only option is to adjust cost or scope.

The project management triangle is a great tool for explaining this to people.  Brandon has a great overview article at his blog:

http://brandoncoppernoll.com/2011/04/quick-look-at-the-project-management-triangle/