I also have back issues that make a good posture difficult to maintain. I've been told that eventually sitting up straight becomes more comfortable, but will have to experiment with that. So I meditate leaning back. If I meditate late at night, I will doze off (actually I'm okay with this since I have insomnia). I don't have that issue if I meditate during the daytime.
has good advice on posture, etc., for meditation newbies.