Do your research beforehand. Look at sky, bt etc and even what virgin are offering new customers.
If they offer lower packages to new customers don't budge, because they can offer it.
Tell them you aren't willing to pay that amount when you can sign up to (reference a package)
Sales teams also like to leave a long awkward pause. Classic sales technique, don't speak first because that gives them the upper hand.
Also don't be rude be nice but assertive, they have targets to meet their bonuses so they need the sale more.
Last but not least if you are an o2 contract opt got the lower speed contract and activate volt and it will double the speed for the same price