Breath and hand, sweat from shirt, water, a sponge with water. You don't need anything else. Tap water works fine.
I happen to know that Der_Echte's real secret weapon is that the oils from his sweat have natural rubber boosting properties.
Try yours out. They may have the same.
Sent from Deep Space by Abacus
Boogar mentioned the salt from sweat. He has somewhat of a point, but the oil from the skin will maintain the rubber anyway. I don't know about any noticeable boost effects though. Maybe with Der_ sweat.
I'm somewhat pragmatic: I barely clean my rubber during games apart from a wipe when it gets sweaty, to get some dust off, and then when I come home I put a little tap water on both sides, spread the water up the rubber face, wipe excess off with my palm then take a non-harsh towel and just towel it off straight off the face. The rest will dry very soon and I wipe some small excess off with my finger and the rest will evaporate.
I was somewhat paranoid about the rubber losing grip when I first bough it, because every single little visible particle in the air would stick to it, but it's not that tacky anymore although the grip is still great.
Then again, it is my first rubber and my technique has improved a lot, so I could just be compensating for less grip. So what do I know.
I've heard that excessive rubber cleaner use will turn your rubber into antispin over some months/years, and that some players purposely do this to get the effect. If that is true, yeah...
I am somehow sure that using special water won't provide any real tangible benefit. Apart from a strong placebo effect. Which could be good in itself.