Clear message: Design Ad copies that have a very clear, succinct message.
Research: Research your network for best placements. If not sure, then place automatically and refer back to click thrus on a daily basis. Exclude placements that look too fishy (i.e. CTR of 100%).
Build on Time and Locality: Narrow down your best audience. Use metrics such as time of day and geographic region.
Keyword Targeting: Content network keywords are very different than search network. You need to create a keyword grouping that define a theme. If the page on a network matches your "theme" then your ad will be displayed. So keep that in mind, dont use more than a dozen words to define a theme.
Negative words: Also use negative keywords to filter out bad traffic.
Generate Demand Conversions: Content network traffic is more passive than search network traffic. Often you can increase your conversion by keeping your visitor mind-set in mind. Content networks tend to produce more "research" type visitors, who are willing to listen to your webinar, download a whitepaper, try out a ROI tool, etc.
Say it in 3: Whatever your ad copy is, make sure it has 3 simple words to capture the idea. Less is more in Google Content.
Online Look: Design the ad using fonts, colors, etc that look good for the screen.
Less Flash: Dont be tempted to use too much animation. It looks cheap and distracts.
Change is key: Your content crowd likes change so mix things up every week or so. Don't just give the same ads, or your CTR will drop along with your quality score.