When you start practicing octaves, begin with 5 mins a day (or twice a day) in order to build muscles without hurting yourself. Slowly increase your time of practice, but never practice in pain or for a long period of time without good arches. Be patient!
A good posture gives you strength and good intonation
Your wrist, your elbow and the tip of your fingers are most of the time on the same line.
If your elbow is too low you will try to transmit weight throught your wrist, it will become painful, you will not have a good sound because you won't have enough weight on the strings.
The weight of your arm needs support from your shoulder/your back
--> Be careful with the posture of your shoulder-blades, it is a lot more important than what you think...