I have never had any really bad dirt that would not come off as long as you use the blower often  enough to keep the sensor clean, and avoid situations that will cause dirt to get in esp when changing lens out of doors.. Dirt sticking to the sensor is often the result of the camera getting steamed up and this can be hard to shift.. I would recommend a service centre to do this
 

But I do have the camera serviced every 12-18 months and they do that as part of the job..  

Unit 130 Centennial Park
Centennial Avenue
Elstree Hill South
Borehamwood
Elstree
Hertfordshire
WD6 3SE

Tel: 08705 143 723
Fax: 020 8731 4139

  Much depends on the seals on the camera some are far better than others..  You can get the sensor cleaned as part of the warranty and I  get it done free as part of being a member of the CPS Canon Professional Services..  You can join this if you have enough gear to qualify,  and they do it while I  wait... the cost maybe around 25 pounds otherwise you can check with them.

May not be what you want to do but if you take into consideration the costs v value of your gear it is worthwhile make it a xmas pressy for the camera lol..    I plan to go there soon to get it done once more..  

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt