Unless you have a parasite problem, a good scrubbing is probably all it needs. Hot water and ivory liquid dish soap (the white kind in the squeeze bottle) is what I use. I also have a shark steamer that I use for hard to clean things like branches and vines.
If it's small enough, you can soak it in the tub. I actually hose everything off in my driveway. Be careful not to scrub the screen too hard, or you will damage it (voice of experience, haha).
Someone else might have some other ideas, too.