Apologies if you already know this but I thought it was so important it was worth passing on!
When you change lenses on your DSLR, always:
1. switch the camera OFF. The sensor is an electrostatically charged device and will suck up dust lie Dracula on a virgin. With the camera switched off this will reduce significantly the level of charge.
2. point the camera down as best you can. I know dust blows up too but most of it is coming down – every little helps.
Not a total cure but a step in the right direction.
Banish All Dust Descending Into Every Sensor – BADDIES!!