It's Not Just Static Cling - Nikon D300 Complete Manual

lamp to warn you that it needs to close the shutter.
Nikon's manual claims that you'll have about two minutes
from the first warning until the shutter actually closes. I
think that in practice, the warning is closer to the actual
shutter close, and at least one D200 user claims they got
almost no warning once, though I haven't heard the same
problem from any D300 users. I find that the shutter
consistently closes in a bit less than 90 seconds on my
D300, and it could be battery dependent (i.e. a battery
that doesn't hold 100% charge but only 90% when fully
charged would act differently). I strongly suggest that you
use a Nikon EN-EL3e and not a third-party battery for this
type of cleaning.
Personally, I will only use battery powered mirror lockup
for cleaning as a last resort, and then only after thoroughly
charging a battery. The risk is that you still have your
cleaning tool inside the camera when the shutter tries to
close. This will dislodge the shutter blades, rendering your
camera inoperative. While I and others successfully
lobbied for this feature to be added to Nikon cameras a
few years ago, I repeat, it's a last resort option because of
the increased risk of damaging the camera. Use it at your
own risk.

It's Not Just Static Cling

Dust clings to the filter array in front of the sensor only partly
because the sensor puts out a static charge. The location of
the D300's filter and the Indium Tin Oxide coating on it make
it much less prone to static dust attraction issues than previous
DSLRs. Still, dust collection on the sensor has always been
more due to the laws of gravity and surface tension.
If you store your D300 on its back, gravity will have its way,
and as the inevitable dust in the air settles, it'll settle
downwards onto the filter surface. If you store the D300 on its
bottom, the sensor still manages to "grab" a few small dust
particles due to the mechanics of surface tension. Generally,
dust that sticks to the filter due to surface tension is easily
removed with light bursts of plain air (the blower bulb). If
possible, the best storage position for a D300 is lens-mount
down.
Thom Hogan's Complete Guide to the Nikon D300
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