First you must know why are people paying for ad on such websites. Think of your website as a shopping center. Having an cafe shop in popular shopping center will get you big, big profit. But to enter that game, you'll need a connections, lots of money and reputable name.
Reason why people will pay high mortgage at shopping centers are clear: lots of traffic and potential buyers. But, ability of the seller to turn that traffic into leads and customers is what really matters. If you're good at that conversion - you'll be able to pay higher price for traffic, as bigger part of it will become your customers.
Your job, as shopping center owner( read: website owner), will be to help your advertisers by any means to convert as much of the traffic as possible. Otherwise, even if your traffic is high you're going to fail.
Therefore, you need to be innovative. Get more people to come by offering coupons, discounts, special offers sent to email etc. Offer additional service for your advertisers - create content for them, review their services and product, create videos and post them on youtube, build community on social websites etc. Offer free printed catalog with content of your website etc. Do everything you can so that your advertisers sell as much as possible.
So you need to be new, innovative, progressive. Move the game to the next level and always ask yourself "Why would they work with me instead of all of the others out there". Once you have great answer to this question in one sentence, the answer that will make them say "I want to try your service, it looks promising" then you'll know that your product is going to succeed.
Hope I help a bit.