First, blow off as much of the dust/dirt as you can without touching the film.
Then, use a film cleaner (e.g. Pec 12, SDS film cleaner, etc.). Anything water based, or containing water, will make the emulsion swell and soften. This in turn makes it much more susceptible to scratching. So... no water.
Use a cloth designed for this, like a scanner wipe or a Pec Pad. Do not scrub -- scrubbing back and forth is an excellent way to scratch film. Instead, start at one end of the film and wipe to the other end with one continuous motion, rotating the pad as you go to left the dirt up and away from the film.
It's not as hard as it sounds. Repeat as necessary.