1. Quality of Your Content: If your site has high quality content that advertisers find appealing, they bid higher for your ad spots - leading to higher CPM.
2. Scale of Your Site: As your site becomes bigger, it gets on more "Whitelists" in large ad campaigns, leading to higher bids. Some large advertisers avoid small sites that they know nothing about.
3. Country: Each country has its own level of advertising maturity. The US market is the most mature. The more traffic you have from USA, the better your performance will be.
4. Ad Formats: Larger ad formats like 300x250 and 160x600 tend to perform better. If you are using older formats like 468x60 or 120x600, consider switching to the newer formats.
5. Viewability: Make sure your ads are seen. Improve the viewability of your ads and your earnings will increase over time. Advertisers want their ads appearing in the most "viewable" positions. By making your ad unit more viewable, your site will be of more interest to advertisers and they will bid more, thus increasing your earnings. Consider placing ads above the fold.
Things that can severely harm your earnings:
1. Banned by Google: A lot of large advertisers put your site on "blacklists" if they see you have been banned by Google. You can check your status here: http://www.bannedcheck.com/ (Please note: Chitika is not associated with this site!)
2. Malware: If systems like McAfee mark your site as Malware, you will get banned by Chitika (and a lot of other networks and advertisers). You can check your status here: https://sitecheck.sucuri.net/
3. Number of Ads: If you have too many ads on your site, advertiser performance will suffer and advertisers will either stop bidding on your site - or reduce their bid.