Implementation of a project is rarely the end of the process. A subsequent release is (nearly) always foreseen, and can even start during the final stages of the current implementation.
Carefully documenting and storing your test cases so they can be referenced, or hopefully re-used.
Developing tests which can be used as regression tests for the next release.
Collating any information you, or the customer, have gained through experience of the testing process.
This may help form requirements for the next release; for example, as the customer's experience of the product has increased during testing.