Great article from knowyourdslr.com about how to remove dust spots from your photos:
If you use a DSLR, sooner or later you’re going to find yourself swapping lenses – that kit lens that came with the camera isn’t going to do all the jobs all the time. That means, sooner or later, you’re going to find yourself facing a problem that photographers have faced since the advent of interchangeable lenses: dust.
Dust is a huge problem: it’s next to impossible to prevent, hard to get rid of and it ruins the best of shots. Many recent DSLRs have self-cleaning sensors to try to minimise the problem, and some can have custom dust maps created so that dust spots are removed in-camera, but knowing how to use the heal tool to get rid of those conspicuous dark spots is a must.