Canon 30D spots

It's 99% dust, especially if it appears mostly in smaller apertures (<f/8). To test it, select a small aperture (e.g. f/16) and take a photo of a blue sky. The spots will show consistently on the same location.

They're hard to clean, so chances are you just missed them. Try again, or, better still, send it for cleaning to a professional.

Dark spots showing up in the same place on every image indicate your sensor needs cleaning. Usually these are most noticeable when you have large areas of a single color. As the previous poster said, a blue sky is a good measurement. Your owners manual will detail the steps for cleaning, but my recommendation is that you take it to a Canon certified dealer and have it done professionally. Unless it's REALLY nasty, they shouldn't charge you more than a few dollars for the job, do it while you wait and you're on your way happy. Some places will even do it free as a way of building goodwill. You CAN do some very minimal cleaning blowing off the sensor with a squeeze bulb, but it rarely has much effect. Whatever you do, DO NOT TOUCH the sensor with a cloth, fingers, cotton swab, or anything else.

You can't avoid dust on your sensor, but you can supposedly minimize it by turning the power off when you change lenses. This cuts down on dust attracting static (or so the manual says). Can't confirm or deny since I never remember to do it.