Do you ever clean the front element of your lens by fogging it up with your breath and then wiping it off with a cloth? If so, you might want to stop -- Nikon says the practice could be damaging to your glass. Apparently human breath contains stuff that isn't too friendly toward camera lenses.
If you have an old plastic kit lenses lying around, something that you are not using for anything serious, you can give it a new life as a macro lens by removing the front element.
Having some dust or smudges on your lens’ front element generally doesn’t have a noticeable effect on …
Canon's 50mm f/1.8 Mark II is a terrific lens for its price, but its build quality definitely leaves something to be desired. Do a quick search, and you'll find legions of broken-hearted Canonites who had their 'Nifty Fifty' split into two pieces after accidentally bumping or dropping it. Flickr user tastygiant is one such Canonite, but he subsequently discovered an awesome use for the broken lens:
Being a geek, I figured I could use the broken pieces in the future, so I shelved it and bought a new 50mm 1.8 Canon lens. One day, while taking shots around my apartment, I stumbled across the broken lens again and decided to reverse the "barrel assembly" onto the front of my intact 50mm. Everything was blurry of course, but I noticed if I got very close to an object the detail came into view. After adjusting the aperture to around f5.6, I had a clear image.
It's important to note that you should switch to Manual focusing and rotate the focusing ring to "infinity".