I use a microfiber cloth that I bought at Staples.
It's $6 US and some change for two cloths.
They work amazingly, and you can just wet it a bit with some water.
Just remember to clean the screen first, then other parts, dry it and do the bottom. (This is because the cloth CAN pick up dust that scratches your screen, but I haven't seen anything like that. Better safe than sorry though, right?)