We have a bottle brush that has been bent to clean most of the cylinder. Every so often we rinse with bleach and then rinse three times with clean water. That seems to keep the crud down. I wish there was a way to keep bugs from getting in the mix.
I used to have that problem where we lived before. Strange, but now that we're surrounded by trees, it hasn't been an issue. For some reason, these guys drink it up so fast I'm cleaning it and refilling it to often for bugs to land in it!