Maybe a stupid question,but,better stupid then phoneless. :D
How and with what can the lense be cleaned with?

On the inside, take it to a nokia service agent todo it. Or you could do it yourself but you would loose warrenty.

On the outside, goto a camera shop or telescope type place, one of these two should have a special lens cleaning cloth and solution. Personlly, I wouldn't bother with the outside as the second I put it into my pocket, it would be dirty again.

Besides these two, there isn't much more you can do. Well, at least as far as I know...