To easily add an application to the list of items that automatically opens upon login, try this...
Control- (or right-) click on the application icon in the dock, and check "Open at Login." This works with both running and non-running applications in the Dock, but does not work with applications located elsewhere, including the applications directory.
[robg adds: This is a simple hint, yes, but it's also a great timesaver that you may not be aware of. In 10.3, it took a number of clicks and drags to add a program to your list of login items. Now it's a simple control-click...]

