Tom brings up a good point about using compressed air----if you try blowing out the dust, the dust has to go somewhere and you don't want it in your dark room!
If I absolutely have to use compressed air, like if you buy holders that were stored in King Tut's pyramid for 3,000 years, take them to a gas station and use the tire air, them bring them home as vacume them thoroughly like a good scout ;-)
Other thoughts:
Some dark slides are made out of stuff that dosen't like water. For this reason I don't give them baths.
Light traps also trap dust. This is one reason why I prefer the wooden black Eastman Kodak made by Graflex ones with the light traps that your can unscrew if the shop vac approach proves futile.
Zip locks are the way to go. If static electricity is a problem,the anti static bags are the way to go, but it hasn't been a problem for me so I get mine at (ugh) Wal mart. For ULF size holders, (ugh) Wal mart sells dust proof zippered inner pillow case liners which look pretty classy and work well.
For grime, a cleanser like 409, Orange or Simple Green---or worst case scenario Goof Off helps. Don't spray it directly on the holder, put it on a rag first.
After you've got them to sparkle, a bit of wax along the edges of the dark slides will make them run smoother. Pledge, Butcher's Wax, Simonize, Blue Coral, Bee's wax whatever works---just use sparingly. I rub it in by rubbing the edges between the tip of my forefinger and thumb.
I hope this helps.
), they're excellent for capturing dust and lint. I vacuum old film holders, wipe them down with an ammonia-based cleaner (Windex), and then run a Swiffer cloth through the light trap. Once they're clean they go into a zip-lock bag.