I have a Nikon D300 and in the past couple years I've found that it seems to require sensor cleaning very frequently, something like once a week or more. I've been dry cleaning with a combination of ultrasonic cleaning and a lens pen quite successfully on each occasion, but then the dust comes right back within a few days. It shows up in pretty much any image with bright uniform surfaces like the sky, small dark dots or threads. The mirror seems to have a way of finding new dust just as frequently, making the viewfinder preview quite dirty. I'm reasonably careful about never leaving the cap off, changing lenses quickly while orienting the camera upright or face down, but it doesn't help.
I suspect what happened is that for a couple years it was stored in open air (no bag/case) in my bedroom, which tends to get quite dusty, and dust got inside the body during that period and is just shifting around as the camera is moved. But I don't have any idea how to clean that dust away - it seems to be inaccessible without completely disassembling the device.
Also, I just upgraded to a D800 and would prefer to avoid this happening again if I can. Would appreciate any tips on either topic. Thanks!
Edit: In case it's relevant, my lens set is all Nikon circa 2009, an 18-55mm f/3.5-5.6G, a 70-200mm f/2.8 VR, a 105mm f/2.8G macro, and a 50mm f/1.4D.