There are some important questions you should ask when deciding which display network to use.   I have compiled a list of the questions to ask during your search:

1. How many sites do you have? Samples?
2. Do you have any exclusive sites?
3. Do you work directly with publishers or are you a re-seller?
4. Do you have any custom filters or filters that we would be interested in?
5. Do you do category, content, behavioral, and retargeting?
6. Do you allow rich media? What % of sites? What provider?
7. What kind of pricing models do you have? CPM, CPE, CPC
8. Who would we be working with? How much experience do they have?
9. What does your reporting look like? How often do we receive? How quickly can we get it if we need it on short notice?
10. Can you track view throughs?
11. How large (file sizes) can standard flash banners be?
12. Do you offer non-profit rates or grants?
13. Does the network have e-newsletter ads?
14. Do you offer mobile advertising?
15. Is your behavioral data based on visits to a site? Or specific pages?
16. Do you cap impressions to single users? What is your cap?
17. Agency or client minimums? Can we combine client budgets to meet minimums?
18. Are you growing? What is your growth/expansion plan?
19. Ask for reach based on filters- if filters are specific, what is the universe?
20. Do you have automated optimization? How does optimization work?

Have fun!