Apple's login items are exceptionally useful for launching programs, running scripts, and opening documents, folders, or other resources that are regularly accessed upon logging in. For instance, a common program to run as a login item is an email client such as Mail. Adding an application to the login items is as easy as right-clicking it in the Dock and choosing "Open at Login" from the contextual menu (in the "Options" submenu in Snow Leopard), but to add other items you must use the Accounts system preferences.
Sometimes, there may be problems with login items, where either items will not stay in the list, or cannot be removed from the list. The login items are simple alias references in a plist file, and adding and removing them from the list just updates the file accordingly. As with many faulty settings, corruption in the property list file can result in settings not sticking, so if this is the case with login items, then try removing the plist file, which is the following:
Before moving this file, be sure to make note of your current login items, since removing the file will clear the login items list. Many times applications will have helper applications start up in the login items, which are required for proper functionality. While starting the application itself in many cases will restore the helper application in the login items, in some instances you may have to add it back manually.