I advise my students to practice in standing position as often as possible! If you practice sitting in the chair, the position of the left wrist is less stressed than in standing position.Make a quick test - Catch a bare chord while sitting, then stand up without letting the chord, and you'll know what I am talking about. Only if you want to sit on the stage, which really isn't very rockiness and cool !
When you stand, pay attention to the spine, always in a flat position !
This is a GREAT point and something I try to stress over and over. Your sitting and standing positions should (IDEALLY) be as close as possible. This isn't always possible, and sometimes, folks just want to play with their guitar around their ankles because, let's face it, it looks cool!
But try this just like Sinisa says. Your hand position should be the same sitting and standing, if it isn't you probably need to adjust your strap. Don't make it a necklace, but try to keep it close as possible to sitting. Otherwise, you will not be able to play as well standing as you can sitting.
The classical position is worth checking out as many people find it very comfortable. I don't use it however.. I play 'side saddle' as Todd calls it when I sit.. but I rarely do that unless recording ideas for something in a hurry.
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I actually started playing "Side Saddle" and only changed when I finally took some Classical Guitar Lessons and went to play in front of my instructor and he chastised me pretty harshly about about poor posture/position/parentage/etc. I was having trouble playing a piece and when he saw my sitting position he was like "AHA!" once I changed it, I could play the piece just fine. Still, different strokes for different folks and some folks just prefer playing side saddle.
Playing standing up is a GREAT idea. It's what you are really heading towards anyway as the reason we learn all this stuff in many cases is to be able to play it live! That means standing up, (pretty much). So as is mentioned further down in the thread, try to find a stand up/ sit down middle path to where the guitar is as close to the same position as possible, sitting or standing.