Check out this write up by 'Leslie Segal', President, Testware Associates, Inc.,on 'Key Considerations in Performance Testing':
http://www.testwareinc.com/resources/articles/
Key_Considerations_in_Performance_Testing.pdf
• Determine the performance testing objectives
• Set pass/fail criteria
• Establish profiles of expected real users
• Determine the proper ratio of real to virtual users
• Create test scripts and executed
• Gather and correlate the data from the test tool and Web/application/other servers.