(assuming Visual Studio 2010)
On the "Web" tab of the project properties, change the "Auto-create port" to "Specific port" and enter the port that you want to always use. 
If possible, you can choose to use the local IIS server.
Update
If you use the Web site project instead of the Web application, this option is located in the project properties window instead of the properties dialog box. Check out the Use Dynamic Ports option here: 
source share