from the Kodak book "Bigger and Better Enlarging"...
Make a thin 'soup' of talc or magnesium carbonate in Kodak film cleaner. The particles will be held in suspension by the liquid. Place negative to be printed on a flat, clean surface with the emulsion side down. Dip a tuft of cotton in the suspension and rub it lightly over the back side of the negative. The liquid evaporates in a few seconds leaving a thin, even coating of the talc particles. Use a camel's hair bursh to even out any streaks or concentration of the particles....Talc particles are small and will not appear in prints made with diffusion enlargers."
Can't say that I've tried it myself. I have a small stash of anti-offset powder (from my prepress days) that I sometimes use - a tiny 'puff' over the neg is virtually invisible, but cuts the newton rings. Less is best