The best thing to do is experiment. With AdSense you can place your ad's into 'channels' which you can monitor. So try out a few different layout patterns and styles using the channels to get an idea of how effective things are.
The key figure is the clickthrough rate and the earnings per click, so keep an eye on these. AdSense also offer 'linkunits', i've implemented these two months ago and now they generate about 35% of my revenue... not bad considering they are only placed on a very small portion of my site compared to standard ad's.
However the best advice is to keep messing about with layout and tring things using the channels; every site is different so you'll need to see works for your's.
Heres a few links worth reading:
http://www.jensense.com/ - a very popular blog about adsense; will keep you informed
http://www.webmasterworld.com/forum89/ - an excellent Adsense message board; probably the most active there is