Dimebag Darrel once said 'you don't go to war sitting down' and he said he never played sitting (if you know the guy's guitar shape, it's no wonder!)

You can see the guys that have practiced a lot sitting down, their guitar is right below their chin whan they stand up!!
I had a hard time at first, but since I work with kids, I can now slong the guitar around my arm so that it's in my back, clap my hands so that the kids get the tempo, make it come back and hit the chord that's supposed to be played... And it just came by getting used to play standing up...