What are some reliable methods for validating antibodies?

In order to validate antibodies, it is important to prove specificity and sensitivity. To test for specificity, the antibody can be tested both on cells having the target antigen and cells with the antigen knocked out. Protein-specific index arrays containing cell lines that express the target antigen in different amounts can be used to test antibodies for sensitivity. It is also important to note that the antibodies should be tested in the application that they are to be used (eg. Western blot, flow cytometry, IHC, ELISA, etc.)