I use a sponge with water. It works fine. But I think all the other methods including sweat work too. I have heard people say that you should not use water because it damages the rubber. Have not found that to be true. I have heard others say you should not use cleaners because they damage the rubber. I have not found that to be true. I have heard sweat is no good for similar reasons and have not found that to be true. I have heard baby oil, vegetable oil, olive oil and WD 40 can restore the grip to the topsheet and I have also heard that these can cause the rubber to break down. Well, the oils work for making the topsheet grab more and for making the sponge expand. I don't know if they shorten the life of the rubber though. For me, they have made it so that I can use a rubber for a little longer, but I only use oil or WD 40 when the rubber is starting to be dead.
One more method works well for during play: steam from your breath and the palm of your hand. But all methods seem to do an acceptable job.