Okay, a non-native english speaker here as well. The biggest boost that I got in my score was by increasing my english vocabulary.
I used the following resources:
1) Barron's 4000+ master list
2) myvocabapp.com -- this was the most helpful!
I found that Barron's list helpful, but I need some form of cover-and-recall way of learning (ie. flash cards). So I tried myvocabapp to help me fill the void.
After using myvocabapp, I was able to plot my progress and better understand where I lacked most and quickly picked up there. 4000 words in 2 months!
Anyway, also make sure you don't spend too much time doing vocab and also keep reading to boost your in-context vocabulary.