I went on a trip recently and it was the first trip where I had my EOS 30D with me to take pictures with. Half way through the trip, I began taking pictures of the scenery including the sky and clouds. I uploaded the photos to a computer to look at it that night and noticed some "smudges" on the pictures. It turned out that some dust and lint fibers has gotten into the camera including the camera sensor. Never having had this problem with my film SLR, I didn't really know what to do. The only cleaning item I had with me was some Kodak lint-free lens paper (which I suspect caused the lint fibers to get introduce into the camera in the first place when I used it to pick up some dirt that got onto the view finder).
I went online to find out what to do. The first page I found was Bob Atkin's article on CMOS sensor cleaning for the EOS 10D. It is a good article that described the problem, how to detect whether you have dust on your sensors and ways to clean the sensor. Of course, reading the article also scared the heck out of me 'cause I would rather not send my camera off to the service center, try to clean the sensor myself and risk damaging the camera, or just pretend the problem doesn't exist.
Regardless, the first thing I wanted to do was to check to see where all the dust particles are and instead of taking another picture (it was already night time), I just took an existing image and set the contrast to be high to make the dark spots more noticeable. Then I took off the lens and with the lens paper I carefully had it pick up just the two major pieces of lint that was on the sensor (Go to the 30D's menu and next to last is a "Sensor Clean" option that will close up the mirror so you can see the sensor but make sure to turn the camera to "Off" when you're done otherwise the cover to the sensor doesn't close.). I barely had the paper touch the lens for fear of causing scratches.
When I got home from my vacation, I got myself a blower bulb and proceeded to pump as much air into the body in hopes that it'll get the dust particles off the sensor. The article felt that this method does very little or would just moves the dust around, but I noticed that on the 30D the walls of the lens compartment has a little bit of stickiness to it which would allow it to grab onto any dust that flies around. After about 10-15 blows from the bulb, I was able to get the dust off my sensor and now everything looks good.
During this experience, I learned a few things. First, dust is common especially with digital SLRs since it doesn't have film that is rolled along and pushes away dust. Second, go with the blower first and don't give up after just a few tries. Some people suggests turning off the camera before switching lenses. I'm not sure if that does anything, but why take the chance... I'm fortunate that the dust that got on the sensor wasn't too stuck on and the blower method was enough.
Finally, go to YouTube and search for sensor cleaning to see actual videos of people cleaning their sensors to get an idea of what to do.