I'd advise NOT cleaning it after EVERY use. Clean it when it needs it - dust, for example, is easily blown off with a rocket blower and/or with a soft brush. No big deal. But I wouldn't be using a solution or friction in general unless there's something on there that will degrade things. Salt spray? Get it off of there. Other stuff? Not so much.
You will be AMAZED at what does not affect the image. You almost have to try to make something visible. Your water mark, for example, should be invisible except under the most extreme conditions - very bright back light, minimum focus, stopped down to minimum aperture (f/32?), etc - and maybe still invisible even then.
I'd be more religious about using the hood - in which case it's fairly hard for almost anything to get onto the front element in the first place.
Water marks I'd deal with using a drop of distilled water and a lens tissue.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!