If it is just dust laying on the film, then using compressed air or canned "air" or a soft brush are good options. The Static Master brushes work well because the short-range radioactive material discharges the static electricity that holds the dust to the film.(posted 8160 days ago)If the film has dust embedded in the emulsion, or other kinds of dirt or stain, then you will probably have to use some sort of liquid. I would avoid a full rewet and wash until nothing else works. With 15 year old B+W negatives a full rewet would probably be safe, but would be a lot of work. With older negatives, rewetting with water can cause damage. After brushing, the next "more powerful" option to try is film cleaner. The PEC-12 Photographic Emulsion Cleaner is popular: see http://www.photosol.com/tech.htm. Kodak is recommending isopropyl alcohol with low water content, 98% or better isopropyl alcohol--do not use the rubbing alcohol sold in drug stores because it has too much water.
You will find excellent advice in Kodak publication CIS-145, "Recommendations for Cleaning Photographic Materials". This is available from the publications section of the Kodak website.