I have some exercises by Godowsky I can give you for pracitsing octaves. I just need to get them off my old computer and I will post them.
Also, practice the top voice alone of octaves with the fingering you would use during performance (5 , or 4 on black keys), and make it as legato and beautiful as possible. It's the outer hand which guides it to the octaves, not the thumb.
Be Back Soon,
Walter Ramsey