I had that problem too. My coach gave me a few techniques that helped. I have only been taking private lessons for about 6 months as well.
1. My arms should feel bouyant, almost like they are floating on top of water.
2. The lifting of the arms should actually come from your back muscles, not necessarily the shoulders. That way your shoulders tend to stay relaxed. I imagine wings unfolding.
3. To keep my frame erect the center needs to be under control. With this part she told me to imagine that I am closing my rib cage together and pressing my belly button into my back.
4. Well, I'm not sure exactly how to say this, but...keep the bosoms up and out....This helped me more than anything with standing straight and tall.
5. I have to remember the narrow base, wide top theory and "stay in the man's hand". Basically the hand on my shoulder I should strive to stay in contact with. This keeps my body and head from dropping into the man's dance space.
I know this might sound silly, but I learn better with examples like this. They may not work for you if you don't have the same learning style and since I am a beginner, there may be other more advanced dancers with better advice.
Good Luck with your dancing.