I ditched the yellow sponge thing that came with the bottle- around here there are too many potholes and bumps to knock it out. The BEST thing I found is those shower loofas/bath thingys. Don't know what to call them, but mine is white, has a rope on it (for easy pulling out of the bottle) and is the same material as the yellow thing that comes with the bottle. It never moves, and is just as easy to pour liquid over. And also- since I lost my straw a couple times when hitting bumps, I found that clear plastic tubing from Lowe's is a great replacement (and at $.25/foot) it's a very cheap fix.

Man- I lose a lot of stuff on the roads....