It is debatable if the verbal part of the GRE exam matters much in your physics PhD application - but if you are determined to improve your score, I have found something (which is free) that I am pretty sure will help you improve your vocabulary a lot. It is called the Mnemosyne Project: http://www.mnemosyne-proj.org/
It is basically an intelligent flashcard program which optimizes your learning time by showing you flashcards from a database of your choice at carefully chosen times.
I have been using it for a few weeks to learn spanish vocabulary, and have found it ENORMOUSLY useful.
In principle you could use it to memorize physics equations too (you can use latex with it), but it would take a long time to create a database of physics formula, whereas there are ready-made databases of GRE style english vocab which you can download from the site.