You can check out my sticky about washing leathers
If it's not THAT dirty, light soap water and a rag or sponge, like you said, then clean off the soap with clean water. After it drys, give it some leather grease to keep it soft and to make it last longer. You can get leather grease in any leather clothes store and some MC accessory shops. I bought a 1kg (2.2lbs) jar of that stuff, lasts a really long time.
SVS. And if at all possible, try to get the bugs off sooner rather then later. It's easier and decaying bugs have an acid effect. It's eaten paint finish if left on long enough. Can't be good for the leather, or the helmet visor. I always clean them off the bike and the gear before putting the bike away.