This is a great thread. It's interesting how different our physiologies are.
I definitely use an enema bottle, but like others here, I am wary of the chemicals, so I just use warm water. I do this hours before I might bottom, because you may think you're all cleaned out when you're not really. As someone above mentioned, if the guy who is topping is well endowed, I need a more thorough cleansing (maybe it's what happens if someone opens up that second sphincter) than if a top is more modestly endowed. Of course, unless it's a regular FWB, one doesn't necessarily know these stats in advance.