You probably had at least some of the dust spots when you bought it new, but didn't notice.
Being careful changing lenses (in my opinion) will at best reduce the problem to a small degree - it won't eliminate it. If the problem is truly dust - the particles that you sometimes see floating in the air in a dark room lit by a single shaft of light, then these are unavoidable because they float and move about with the slightest air currents.
As soon as a lens is removed, this type of dust is floating in the air into the mirror box; when the mirror flaps up and the shutter curtains open and close, air currents are generated, and during exposure, the "sensor is exposed" to the dust. No surprise here.
Another source of dust is from the moving parts themselves, wear bits plus lubricant being flipped off moving parts during mirror and shutter operation - these too are unavoidable.
Conclusion: "dust" on the sensor cannot be avoided - it will happen. And it will happen regularly. You can't prevent "dust", you can only remove the inevitable with a routine cleaning program. In effect it's not unlike one's hands and face - eventually they will need washing.
Solution: Don't rely on "professional" help. If you send your camera to be cleaned every time it's required, you will seldom have the camera in your possession. The only practical solution is to learn to do it yourself.
There are numerous ways to learn this; reading, asking questions, etc. Here are some starters:
http://www.dmcphoto.com/Articles/SensorBrushes/
http://www.cleaningdigitalcameras.com/index.html
http://www.bobatkins.com …utorials/sensorclean.html
http://www.leggnet.com …ensor-of-digital-slr.html
http://www.tribcsp.com/~sigma2/SensorDust01.html
EDIT: By the way, I've had my 30D for just one year, and I've cleaned it in the order of 15 times. Twice a year just wouldn't be adequate.
I would say that it was worse at first, but gradually is taking fewer cleanings - I suspect because the lubricant isn't being thrown off the moving parts as much.
Just for the record, there are reports of sensor dust on cameras that have never had the lens off.
