A lot of guitarists wonder how high or low their guitar should be when they are wearing a strap. Peter Vogl has a quick tip that will help you find a comfortable playing position.
While sitting in a chair at your normal playing position, check and see where the guitar is in relationship to your chest and arms. When you stand up, you want the strap to hold your guitar in the same basic position. This is the optimal height for your guitar to sit at if technique and comfort are your primary concern. If your main objective is looking hard core, you can just let it hang as low as you want.
(Strap Photo via Original Fuzz)