Originally posted by Sagitta The best advice I can give to back this up is to ALWAYS use your rear cap. Don't leave a lens just sitting around because you'll get dust on that rear element and unless you clean that lens before placing it on your camera, a lot of that dust will then transfer to your sensor and you'll be screwed.
I think most people wind up getting dust in the camera because it piggybacks in via a dirty lens than anything, since a lens swap is (usually) a quick enough affair you won't have tons of dust fly in during it.
+1 - dust on top of the front element, while not desirable, at worst
slightly softens an image - nothing really noticeable. Rear element dust can make an image 'blotchy', and as Sagittia points out, you are now deliberately placing dust inside your camera body where it can do much worse to spoil your photographic experience.
I like to store my lenses front element down in my bag. In as clean an environment as possible (meaning yes I change lenses outdoors, but not in poor conditions), I remove a rear cap, inspect the rear element, use a blower* on the rear element if necessary, remove the lens from my camera, transfer the just removed rear cap to that lens, remove the replacement lens from my bag and turn the lens rear element down, put the now capped lens in my bag, then mount the replacement lens on my body. The whole process takes seconds, and the only time I have two lenses in my hands I have a hand dedicated to holding one lens.
The nice thing about rear lens caps is every K-mount rear lens cap fits every K-mount lens. Okay, old caps get mixed with new caps in the field. So what? You can swap them later if you are a purist about which cap goes on which lens.
* Giottos makes several size rocket blowers. I have the big one we typically see with the rocket-fin legs in my office. I have the smaller size without the fins in my gadget bag - more so, you can remove the red nozzle from the smaller blower and reverse it inside the rubber bulb so it fits better in the bag. I have attached a couple scraps of coffee filter material to filter the air coming into the bulb, and I always flush the bulb with a few good squeezes before using it on any part of my camera gear.
I have sensor cleaning brushes carefully stored in a sealed plastic bag. I'm guessing they will work based on other users comments. So far I've never needed anything more than the camera's own built-in functions and my Giottos blowers - and the above handling procedures.