JuicyAds is the only network I now use, and I've tried the lot.
Now my link is out the way, here are some tips.
Make sure you buy a spot on exactly the right niche. Drill down their categories and look at all the sites in that category, new sites tend to undervalue their spots, so if you try them for a week and it looks promising, make sure you book for a month. Webmasters can raise their prices at any time once they realise what they're actually worth.
If you're buying run of network ads, make sure you target the countries you really want to. I almost never buy anything other than Tier 1 traffic, although have had some success with the Tier 2. Tier 3 is mainly for when you want to bump your traffic stats up and not looking to make sales directly.
Also on the RON ads, fix your max daily spend. If you don't know what you're doing and are targeting all the wrong countries and set your cpc too high, you can quickly drain your account.