"Dust-free" and "lens-change" are mutually exclusive. That's just the way it is!
All you can do is try and change lenses and quickly and cleanly as possible (e.g. hold the body facing the ground). Or, you could look a complete twat and use something like this: http://www.morco.uk.com/latest/camera_bivvy.htm
but I seriously doubt it's of any real benefit.
It's one of those devices that are aimed at a certain market - like those brochures selling all kinds of handy, labour-saving devices for the home that you never knew you needed till you saw it in the brochure.
For me, a simple rule of thumb is - do professional photographers use this? If the answer is "no", then I'm pretty sure I don't need it either.
IngemarE From Sweden, joined Mar 2005, 285 posts, RR: 5 Reply 10, posted (6 years 4 months 1 week 6 days 9 hours ago) and read 1279 times:
I have used a plastic bag from one of the local grocerie shops. Works well, if you happen to be in a "contaminated" enviroment.
Especially if you rub it against something first, creating a bit of static electricity in the plastic, which will attract any dust-bunnies that inevitably will have found its way into the bag whilst fiddling around getting the cam&lens in there.
Cost; 1,50 SKr =1,395€/2,1£/1.035$ (Todays exchange-rate! Note that rates can vary from day to day, making your investment more, or less, costly. )