Learning to use custom loops is a must if you want to get beyond the basics in customizing themes for WordPress. This guide is clear and concise and even includes info about using the custom loop in the sidebar and in the Thesis theme, as well as creating a related posts section in your theme. (For the record, if I’m doing much with this kind of thing in the sidebar, I usually install Justin Tadlock’s Query Posts plugin which is seriously the most awesome plugin I know of.)