I don't use soap but I do put my phone under the tap if I dropped some sauce or something on it.

On a serious note, why not just use the lens polishing cloth you get with glasses? If it's good enough for glass that would sit right in front of your eyes constantly, it's good enough for your phone.

I just rub it against my pant leg. For glasses I use little alcohol wipes because the thick grease film drives me nuts.