A serious tip, along with the above 'lie about your income' if they think you're unemployed with no money, they may even take you off the list completely.
I get a few calls with people trying to make me switch my internet, and one was from TalkTalk, the Carphone Warehouse, and when he said it I actually laughed spontaenously down the phone, and had to recover and say..'sha..sorry, carphone warehouse..and internet...no thank you.' As I knew of the 'quality' of their service. (Lets just say they took over AOL in the UK and managed to make AOL WORSE)
I often just take great pleasure in telling them the truth when they say' and how much do you pay for your internet?' telling them, about £6 a month for unlimited 8mb, and they just give up, honestly, at least half just go 'ohh..ok..well, thanks for your time.' with the others rying to sell me a worse service for more money, there's optimism.
So, I've started using the same tactic for all calls like that 'How much do you pay for your home insurance 'Oh £3 a month' 'what are your quarterly energy bills currently 'oh I pay about £20 a quarter all in' and they generally piss off, knowing they're not going to profit from me.
In the end tho, it's a recession, and they're stuck in a crappy job taking abuse all day, and I'd rather not pile on the misery, just instead making it clear I'm of no use to them or their shitty company.
If only more people had the common sense to think 'hey, if I need any service or product ever, I can go shop around and find a good deal for me, instead of waiting for some muppet to sell me one.', if that happened we'd not have telemarketers.