In article , RJH wrote:

Spots (dust spots anyway) are more likely to be on the sensor than the
lens. It is worth learning how to clean the sensor if you are using a
DSLR or a mirrorless system.

Please excuse the hijack - any top tips? My Canon 40D has some now
irritating spots of muck somewhere - the sensor I think.

start with a rocket blower:
http://www.giottosusa.com/rocket-blasters

that should clear all but the most stubborn dust.