Digital Camera Cleaning Techniques and Products:

A dirty sensor can only result in imperfect images - little dots, or smears that aren't on the lens.

If you change lenses in anything other than cleanroom conditions (and who has a cleanroom available when they need one?) you're running the risk of getting a few spare specs of crud on the inside of your camera.

Of course, there are a few steps you can take to mitigate that risk somewhat, such as:

• change lenses as quickly as possible
• find a sheltered area (away from the windy beach)
• carry a plastic bag to do the change in
• keep the camera body pointed down so that crud can't fall into it
• make sure you use lens caps any time the lens isn't in use

However, I reckon it's almost inevitable - eventually, no matter how careful you are, you're going to get a spec of something onto your sensor, and when you do, it's almost guaranteed to show up right where you don't want it to.

Then, you're going to have to look at cleaning your sensor. Hopefully, you'll have bookmarked this site, so you can find out how to safely go about it...