I usually have a brush with me for dust on the outside of the lens, but that dust doesn't show on the picture anyway, I think.

When I switched to digital this spring, I thought I finally would get rid of the dust on the scans. Of course I did, but I didn't know about sensor dust, or dust on the glass in front of the sensor. It shows on pictures taken with small apertures, especially with short focal lengths. But it seems that blowing air helps a lot. I use a rubber balloon meant for earwax removal. My camera also has a feature for shaking the dust off, but it doesn't seem to be as effective.

Methods for cleaning digital cameras.