In general the best cold calling script is a "warm" call script. What I mean is that you should have some connection to the person you are calling even if it is just the fact you have sent them a postcard.
But if are going to go the pure cold call route, the best advice I can give is to be polite, professional, and direct. As soon as they answer the phone they know it is a sales call, so don't try to make it anything else. Open with something like:
"Hi, this is Ben with ABC Insurance, I'm calling about your medicare supplement insurance policy. Would you be interested in lowering you premium significantly without any change in your coverage?
If they are open to talk, go over how medicare plans from different companies all have the same benefits, how any med supp is accepted by providers who take medicare, and why paying more for a plan doesn't get you any "extra" protection.
From there it is a matter of being personable and professional, so they are at each talking to you. Hope this helps!