Since WordPress decided to include the Widgets Plugin with the release of WordPress 2.2, I have converted all of my blogs to Widget ready themes. Widgets make it extremely easy to manage your sidebar(s). If you have WordPress 2.1 or earlier you can still download the Widget Plugin from Automattic and follow their easy instructions to get the widgets to work with your blog.
If you download a new theme, make sure to look for one that is already widget ready. There are quite a few of them out there (including this one).