Here's what I do...
1. Looses lens mount cap on new lens and make sure there is no obvious dust sat on the back element (or in the cap for that matter). If there is, remove it and then place cap loosely back on the lens.
2. (With lens facing down), Dismount old lens and place down somewhere.
3. Remove (loosened) mount cap from new lens and place loosely on dismounted lens.
4. Mount new lens.
5. Tighten lens cap on dismounted lens.
6. Look through viewfinder to notice large dust mote/cat hair has settled on focusing screen.
7. Dismount lens again and try and remove with a couple of blasts from a rocket blower.
8. Remount lens and notice there are even more specs on the viewfinder screen.
9. Repeat step 7.
10. Repeat step 8.
11. Tear hair out.
12. Send to Canon for cleaning.