It really is as simple as IMing your AM and asking "Hey, can you beat 1.15 for QH?" They tend to be pretty straight about it, especially if they know you work with several other networks. I'm registered with more than a dozen, but generally bounce between 3-4 main networks, and they know it. If you do volume they'll bend over backwards not to just give you the best payouts, but ensure your questions and issues are dealt with quickly and payouts are the quickest and easiest for you. If you don't do volume, it's still a highly competitive industry and they want your business. W4 and Hydra though in particular, if you drop network names, have a personal vendetta with each other and so work even harder to beat each others.
Just remember that depending on the network, some aren't getting the offers directly from the company that supports it, but may be getting it through another network, so can't match a payout. If it's one of the bigger/stronger networks like Azoogle, Neverblue, Hydra, W4, EWA, etc. you should be able to get fully competitive rates just by saying that you're being offered XX.XX elsewhere and ask if they can beat it (usually they can at least match it).