Whether you’re using a semi professional or fully professional digital SLR, it’s going to get dirty inside and out. There are some useful specialist cleaning companies who can warranty their own work for up to 6 months - Fixation are based in London (0297 5823294), and also the slightly faster and more reasonably priced service from ACS (01953 889324). Both offer rapid turnaround sensor cleaning (£35 - £50) or a full service for upwards of £100.00. Recommended if you’ve got trouble with those ’stubborn stains’! Other DIY solutions include cleaning kits from Just Limited  which are excellent if you’ve a steady hand to clean the sensor within your camera. 
David Webb from ACS says “we normally have a look at the sensor itself under powerful magnification using a strong light, take an image at f11, then clean the sensor, then take several more images to make sure everythings cleaned up.” They have seen 3 or 4 damaged coatings on Canon 5D models as well the 350D, for which the replacement sensor can cost upwards of £700. David reckons it’s possible to clean a digital SLR to better-than-new condition - so there’s hope for those owners struggling with dirt and dust particles.