In late April the Occoquan gets REALLY good. In the back of the river there are Stripers, Shad, and the Snakehead spawn so you can catch them in there during the same time. I do better for stripers in late September through October as long as the water is running from the dam. If the water isn't flowing it really isn't worth fishing to hard.
In the right conditions I have caught about 50 stripers in a 4 hour slot fishing the Occoquan River. There is also a few different places that you can put in for free if you don't want to use Occoquan Regional Park.