I only use BT for basic landline access, so there's not really much they can knock off my bill.
I know that my ISP is far more expensive than most but they provide an excellent service (because they mainly serve big businesses, such as BAE Systems, rather than domestic customers, so they're used to customers expecting nothing but the best from them).
I do it every time my contract comes up for renewal. It always pays off. I've just done it again, to avoid the latest increase in charges. I negotiate satisfactorily with Sky too. But, when I tried to negotiate with Eon, last time my contract came up for renewal I ended up with a flea in my ear. Did renew, but prices have gone up so much, when my contract comes up for renewal again I am going to switch.