Here is how I did it:
Negotiation Research: What special offers are the competitors offering?  Dish Network is offering a similar deal for new customers.  Sure I don't want to switch and think their service is weaker than Comcast, but Comcast doesn't know that.

BATNA: What is my Best Alternative to No Agreement?  Well I already have Comcast service and I am happy with it, I am just paying a bit too much for it.  So really my alternative to having Comcast sign me up for their program is to maintain the Status Quo.  It is not like they are going to cancel my service for trying to negotiate with them.  I really have nothing to lose.  They don't have to know that I really don't plan on switching to Dish Network.

Making the Call: 1-800-COM-CAST (I hate mnemonic phone numbers!) and ask for customer service.  Once I get a representative I tell them how much I like the new offer and that I am interested in signing up for it.  I already know I don't qualify but I want them to starting trying to help me out.  The Customer Service Representative(CSR) tells me that the offer is only for new customers or existing customers that don't already have all three services.  I politely ask if there is anything they can do.  Of course the answer is "no", but hey the first line usually doesn't have the authority. 

Escalation: I then ask who would have the authority to make a change, especially if I am thinking of changing my service provider?  The CSR immediately transfers me to a resolution manager.  BINGO! Someone who can actually help.  Ok so these resolution managers(or whatever they are called) actually get paid to keep you as a customer but also get a commission for not giving in to your demands.  If they give in the make a less commission, if they lose you as a customer they make less than if they give in.  So their ultimate goals is to keep you at the status quo but give in before losing you as a customer.  I start all over with this Resolution Manager.  I tell them I really like the offer and ask if I qualify.  Again the answer is "No, it is only for new customers."  I then stay really polite but mention the offer from Dish Network.  I then ask them "Why shouldn't I make the switch to the other company?"  Ball game changes! 

First Offer: They offer me the plan without some of the bells and whistles.  The Resolution Manager is at this point trying to hang on to some of her extra commission for not giving totally in to my requests.  My goal here is to stay nice and keep her happy with me but be frank about my willingness to leave(not really.)  I tell her that I have some friends that really love Dish Network and that their offer is really enticing.  I also tell her that I am a long time customer and have always been really happy with the service that Comcast has given me.  Hook Set!

Next Offer: She offers me the plan as advertised.  BINGO!  I have achieved my goal.  I thank the rep for her help(Always have your opponent leave happy) and tell her how happy I am to be able to keep Comcast. 

Have you ever had to negotiate with your Cable company?  What was your experience?