The trombone is really just a long tuning slide that you can use to play different pitches. This page shows you all the pitches you are able to play with any given slide length. You'll notice there's a pattern in the notes; the intervals between them are all the same. This is called the overtone series. All brass instruments work based on that pattern.
We have talked to many trombone players and teachers about this; what's the best way to differentiate second and third position? Where exactly is fifth position? One thing everyone agrees on is that trombone players have to learn to use their ears, not their eyes, when finding the right slide position. (This is the exercise we mentioned on the trombone intonation page, if you've been there first.) This exercise is from the Remington book, which we mention several times on this site. One of our favorite exercises from this book- and one of the most famous- is this descending pattern that is very useful for practicing accuracy in slide positions. Remember, you're basically holding a tuning slide in your right hand; it's important to listen to yourself as you play, in case your slide lands a little too long or to short for the note you're aiming for. Here's a sample of that exercise:
For practicing slide technique...
Check out David Vining's book, Daily Routines for the Student Trombone Player. (There are several other versions of this book if you are looking for something more advanced, or want to work on your bass trombone skills.)