Have a friend push downward on your arm while you try to keep it level. Normally, you should be able to resist. Then bend forward and try to keep your arm up. One problem is that holding an erect position eventually tires the back muscles. Dr. Waverly Chin, a Hong Kong chiropractor, advises getting up and walking around every 45 minutes or so to allow your muscles some relief.
Many office workers position special cushions in the small of their back to provide some support.
I've used a lot of them, and they are not all that comfortable because they push your body up. It doesn't feel natural. In my view, it's much better to buy a decent chair. There are other things you can do to minimize back strain. First, for men especially, remove any objects such as wallets or change from back pockets.