This tip actually applies to traffic from any channel but its definitely something you can do to improve your return on ad spend and justify your cost-per-conversion for ppc ads.

Before killing a costly adgroup, make sure you’ve offered everything searchers may be looking for when they click on your ad.  For example, try selling different variations of your product… different colors, different lengths, different brands, etc.  If you’re only offering one or two varieties of a product, a searcher will be more likely to go with a competitor that offers a more depth product line.