Dust on a sensor is a fact of life with digital photography
Get used to it.
It will get into the camera no matter how careful you are.
Using a zoom lens is one of the most effective ways of getting it there.
Some lenses, like the 100-400L are worse than others.
The 100-400 is a bloody good lens but it's like a bellows.
Anyone who has one, try this - take the end cap off, hold one hand over the end that attaches to the body and zoom it in and out to it's full extent. You'll feel the hurricane that it produces.
That wind also contains miniscule particles of dust that land in the camera box and then eventually on the sensor.
I was once complaining about dust to the tech who does my sensor cleaning and he said there is only one foolproof way to keep the dust out of your camera, and expecting a pearl of wisdom, I asked him "How is that?"
His reply was to put it back in its box, in a cupboard and not use it - simple.
From that day forward I gave up worrying about it and just accepted it will happen.
I use a rocket blower regularly and get the sensor cleaned by the tech about every 6-8 months as it needs it.
I shoot plenty of motorsport, particularly rallies. Rallies equal dust. If I have to or want to change lenses all you can do is pick when to do it to minimise the chance or amount of dust getting in. It's either that or not get the shot.
A small to medium amount of dust won't damage your sensor or camera. It's more a pita because at small f stop's it shows up more and you have to clone it out.
Way too many people get hung up on dust - DON"T.
It's a bit like dust in your house. It gets there no matter what you do or how clean you think you keep everything.
