I read somewhere, maybe on here, that some people are using a real cotton tipped q-tip dipped in eyeglass cleaning solution. I haven't had the need to clean the lens but that is what I would use. Remember, even truly cotton q-tips are not ultra-soft and if you press too hard you can potentially scratch the lens or the cover over the lens.
I use a microfiber cloth. Seems to work fine. I don't use the one that came with the phone, it only seems to smear the smudges. I got a big pack of yellow ones at a local Costco. They do a great job of cleaning the iPhone screen and also make my MacBook screen look new.
I use a 3m high performance cloth. I got it with an eyeglass cleaning kit. When even that thing needs cleaning, I throw it in a hot washer load and dry it in permanent press. Then it cleans like magic.