To get rid of dust or debris that may have accumulated on the slide with the passage of time, spray a small quantity of compressed photographic gas on the slide. There many brands of compressed photographic gas available at a local camera store. In order to choose the right brand, just make sure the brand you use is perfectly safe for use on negatives.
Gently wipe clean the surface of the slide with a clean piece of lint-free cloth. Make sure that you do not apply too much force or scrub the slide because doing so will leave irreparable scratches on the slide’s surface.
Alternatively, you can use pure alcohol applied on cotton balls to remove dust and debris. Do not use rubbing alcohol because chances are that it will contain water, which in turn will soften the dye on the slide and thus remove your picture.
Purchase a bottle of quality film cleaner and make sure that the brand you choose is safe for use on slides. Moisten another piece of lint-free cloth with the film cleaner and softly rub the surface of the slide with it.
Fibres from the cloth may have accumulated on the slide. Allow the slide to air dry and then spray it with compressed photographic gas once more. This will remove any accumulated fibres.
If you so much as suspect that a particular slide will not be able to sustain the cleaning techniques mentioned above, your best option will be to play safe. Take that particular slide to a local camera store and get a copy of the film. Now you will at least have backup even if the slide becomes irreparably damaged when you clean it.