sandpaper was mentioned above, Emory cloth is better. 220 is about as course as you should consider. A steel brush will clean it quickly but will like breakdown any protective plating too. Light brushing with brass brush should be OK. Most contacts are tin or solder plated which protects the easily corroded copper under. Keeping the plating in tact will help the contacts last longer. Contact cleaner and dielectric grease will finish the job.
