I'd agree with Clanad - vinegar (any sort) will remove the tarnish really easily. If you don't have any vinegar, then either try lemon juice or even something like tomato ketchup which contains an acid. Leave on for 5 minutes or so, rinse or wipe off, then gently rub with some brasso (metal polish) or very fine wire wool. On my solid brass letter box and house numbers, I let them soak in ordinary malt vinegar for 5 minutes, rinse off, then gently rub the tarnish off with a brillo pad.... Very little elbow grease required ! (It takes longer to dismantle the letter box etc than it does to clean the bits up !)