There is this stuff you can get at Wal-Mart that really works well called Weiman Silver Cream. It doesn't have any really harsh corrosives in it so you don't have to worry so much if you happen to accidentally get some on the leather. It works wonders. Since tack seems to have more little tiny cracks and crevasses than silverware, I used an old toothbrush to scrub with.
I used it on this tack (the silver was so tarnished it was nearly black when I got it out).
TOOTHPASTE! Someone on the forum reccomended it to someone a while ago, so I tried it on the conchos on Lenox's bridle, and it really shined them up. I put it on a rag and rubbed it in, then washed it out. Worked great!