From LensRentals Blog:
"I [Roger] get asked about 15 times a week, 'How can I get the dust out of my lens?' The right answer is you don’t. All lenses have dust in them and it doesn’t affect the images at all 99% of the time. Even if you clean it all out, it will be back after you use the lens a few times.
There are occasionally times that large dust specs very near the rear element are visible in an image, though. There also is the very real issue of resale value; a dusty lens tends to bring a lower price than one without much dust. The right answer in these cases is “send it in for factory service, they’ll disassemble it and clean it.” Doing it yourself is risky.
After I give all of those answers, a lot of people tell me they have an old lens no longer under warranty, not worth the cost of sending it in for factory cleaning, and they are really handy and want to do it themselves. For those people, we’ve put up this post showing how to get dust out of some fairly easy to reach locations."
See Roger's full guide for cleaning your lens. Be sure to read all the warnings carefully before trying this yourself.