By blending them completely or making the ad color slighly more prominent.... or making them contrast dramatically. It varies from site to site so you need to test. Subtlety is probably the best approach. Dean
Make the text and color look the same, similar like your site content. Remove the border and make sure that the ads are placed on location whereby visitors are more likely to click on them.
You can use a HTML table to place images on top of your horizontal ads to make them look more like content. There was a thread on it somewhere here on DP but I can't seem to find it at this moment.
I try and blend the ads in so that people click by accident. Because the type of people who will use my sites are usually teenagers, they probably won't find the ads interesting and therefore won't click. If you do this then be careful that you aren't misleading, as that is against the Adsense TOS.
try to blend it in your page theme..color blending..but do a research on how to put ads on website, and it should comply with the adsense TOS..