I think it was during MeeGo Conference 2010 when I was first told that I should cover the N900 camera cover's blue and silver bevel with a dark color to prevent them from reflecting the flash light and making the photos worse than they could be. If you don't use the N900 with flash at all, this "hack" doesn't apply to you. Otherwise read on. If you look at the N900's camera cover, you see that the camera hole is surrounded by a blue border on one side, and silver borders on the three other sides:
Now, when you take a photo with the flash, the flash light is reflected, which makes the taken photo to appear as if there was some smoke or fog in front of you. In reality it is the reflection of the bevel. So, take a black marker and take off the back cover of your N900 (you don't want to accidentally cover your camera lens with black color!):
Now carefully cover the silver parts around the camera hole (and the blue part too, ideally) with black color. The result should look like this:
You might get even better results if you take a marker with a finer tip or even use black spray paint or something. If you don't have a marker ready or do not want to paint on the back cover, you can also take off the back cover of your N900 (which has the same effect), but you will probably need a small magnet to trick the magnetic switch into reporting "camera cover opened" to the camera application.