How can you prioritize resources?

In the “Scheduling” tab you can prioritize resources by giving them different priorities as shown on the picture below.

Be sure to use date constraints on project or on your tasks.

For instance, you can give a priority of 1 to your internal resources and a priority of 2 to your external resources.

By doing that, the application will use your external resources only if none of the other resources with priority 1 are available.