You guys have it all wrong. if you're a grumpy customer, they don't want to keep you and will cancel you. honey attracts more flies than vinegar.
I pay $14.99/mo for HSI 6mbps w/ powerboost (25mbps download speeds

).
Here's what you do.
Have Notepad open (or pen and paper if you're old school)
call 1800comcast. choose "new service" as that will get you to a rep the fastest.
when they answer, tell them you want to cancel. they will transfer to a retentions rep/cancellations dept.
here's the important part:
When the retentions rep answers, DO NOT SAY YOU WANT TO CANCEL.
Say "What is the procedure for cancellation?"
They should say "What do you mean"
Say "When I cancel, do I just need to call up and that will be it? Or is there anything else I need to do?"
If the rep just answers your question, thank them for your time and hang up. write down their name and call back. Hang up if you get the same person.
The rep you want will say "May I ask why you're cancelling?" THIS IS THE RESPONSE YOU WANT.
Say "Competing HSI is $15/month" or whatever. They will try to say things like Competing HSI is slower, you have to sign a contract, etc. Just tell them it's cheaper and you simply can't afford to pay your current rate.
At this point they will offer you a discount. tell them you'll think about it. DO NOT ACCEPT> thank them, write down their name, hang up and call back.
rinse, repeat. some people will offer $30 for 6 months, some will offer $20 for 3 or 4 months, some will offer $20 for 6 months, some will hit the gold mine and offer $15/month for 12 months like they did with me.
when you get the rate you're satisfied with, just call back and say you talked to such-and-such and you'll get the rate you were promised.
Obviously this is not a hard and fast guide, don't say things word for word. the gist is you're "inquiring" about the cancellation process(not actually cancelling)and calling multiple times to get the best deal.
BTW this works with the cable portion of your bill too