On a daily level I work with landing pages and I cannot stress the importance of having a good landing page. This goes especially for Google adsense pages, which will not only increase your conversion but also increase your Google adsense account, so you pay less for your keywords.
This video gives you a very good introduction to doing landing page improvements. The bad thing with this video is that the examples they use are very poor. This means that there are a lot of low hanging fruits. Notheless it shows you some of the misstakes to avoid when doing landing page optimisations.