I worked for Sky in retentions for over a year recently, briefly as a team leader (I now work in a similar role for BT).
The best way to negotiate a decent price is to just threaten to cancel it all and retentions will get you a deal. The offers on every account are different based on tenure, if you have been a good bill payer, the package you are on etc. If you are still within minimum term however you won't be allowed to cancel and won't be offered anything.
I will warn that putting in notice to cancel and then waiting for someone to phone you back is a bit risky. If someone phoned in and said they were doing this or had done it in the past I would remove them from marketing so that Sky wouldn't contact them again and plenty of others did the same. Adversely affected our stats regardless so we didn't care.