I've had the mark II version for a couple years. I am careful with it, but use it outdoors, at events, etc. My lens has definitely collected some internal dust and I also have a few specs that have settled near the rear element as well.
I doubt there is anyway practical way to avoid this. The dust has not noticeably affected image quality and I feel the amount of dust inside after a few years of use is tolelrable. With this much glass it's pretty easy to notice even the smallest dust particles...I think it was only a couple days after I purchased the lens that I noticed my first dust spec. It's kind of like a new car though, once you get past that first scratch...you care less and less about subsequent dings
On a side note; someone attempted to steal my 85mm (while attached to a camera) once at a wedding I was shooting. I got the lens and camera back, but in the process it was dropped - the B+W UV filter was trashed and lens barrel has a nice scratch near the front element, but other than that the lens was fine. It's a solid lens and I wouldn't hesitate bringing it outside to play with. BTW the guy trying to steal the camera was not as lucky as the 85mm in terms of damage received.