Yeah.. I remember when I changed my acoustic guitar to an electric.. It was damn hard to play standing. I kept my guitar also below my waste and just like umm.. 6 months ago I lifted it much higher. I thought it looked ugly adn I even asked my friends if it was not goodlooking but they said it was Ok so I continued playing like that and it is much easier now.

One thing you might want to try is that when you play sitting the best place for the guitar is on your left leg (or right if you are left handed) like you played classical guitar, you get the best touch to your fretboard in that kind of position. Now, when you stand up try to keep that same position with your guitar, if it hangs lower than when you played sitting you might want to lift the guitar up a bit. When it is exactly in the same position as it was when you were sitting you have achieved your goal. I think that's the best way to learn how to play standing.

Hopefully you got what I was trying to say there.. :)