Originally Posted by: renyou should practice in the same position as you'll be playing.

For that reason, I recommend playing with your guitar strapped on your shoulder, and high enough that it's not restiing on your leg when you're sat down. That way, it's in the same poition whether you're sat down or standing up...

Absolutely, but also remember to pay close attention to your posture. Make sure that your back is straight at all times and that there is no tension anywhere in your body WHATSOEVER. This is really important as not only will it help you to play easier but playing whilst tensing up can cause serious damage.