Mac,
Really, all I have heard of has been mentioned....rubber mat under the base, LARGE magnet stuck under the heel, and chain around the waist (or any combination thereof).
Ask five blacksmiths what they think works, and you will get 10 different answers, usually!!
I personally believe Rusty is correct, I worked on one for the last four days with a magnet under the heel, and I don't see (or hear) any difference. About the best you can do is find some comfortable ear plugs to help keep your hearing from declining. The cheap little cone shaped ones, with a plastic loop connecting them are about the handiest I have found for forging, as they are easy to apply and drop around your neck when not needed.
I know one smith in our organization that just swears by Fisher anvils, simply because they don't ring. The only reason I like a ringing anvil, is because most usually the rebound is great and will bounce your hammer right back up with little lifting. When it is all working together correctly, there is no better rhythym than that!