We've given you many optimization tips over the years, and, as you know, it's important to consider how your ad implementations affect not only your click-through rate, but your users' and advertisers' experience as well. To support this, we'd like to remind you of the following two guidelines when optimizing your site.
1. Ads shouldn't be placed under a title or section heading in a way that implies that the ads are not ads.
For example, ads shouldn't be placed under titles such as “Dallas Business Opportunities” or “Today's Hot Deals”. Placing ads directly below titles such as these implies to your users that the links in the ads are publisher-created content. The example below shows a placement that does not follow this guideline.

2. Ads should be easily distinguishable from surrounding content.
Similarly, you should not place an ad unit by a group of links that has identical colors and line spacing. Doing so may cause users to think the ad unit is content created by you. In this situation, we recommend using a different color for the ad titles or indenting the ad unit to help distinguish the ads from your own content. This screenshot shows an implementation that does not follow this guideline:

As you can imagine, users who click on ads that they think are publisher-created content may lose trust in your site and decide not to return in the future. It's important to keep their interests in mind, as well as your own.

Also, advertisers can tell which sites have a high conversion rate for them using Placement Performance reports. A conversion occurs when a click on an ad leads directly to user behavior that the advertiser deems valuable, such as a purchase, sign-up, page view, or lead. Advertisers have the option to exclude your site from their campaigns and may do so if it is not leading to conversions.

We believe these guidelines invest in the long-term health of the relationship between AdSense publishers, AdWords advertisers, and your sites' visitors, and that they will help ensure your continued success in the AdSense program.