How can I prevent applications from running on startup?
Please, please, please, please put down the task killer.

It isn't needed.  The operating system knows how to handle and end applications just fine.  Otherwise you are going to waste performance/battery because when you kill a task it is going to re-spawn back up.  And if you  killed a task that was writing to disk (or a database), then you could potentially create corruption.

The only time you ever, ever, ever should kill a task is if it is obviously running wild or hung up (and usually when this happens you get the Force Close screen.  If not, go to Settings --> Applications --> find your app and kill the task there).