only recently have I started to really sit and start to memorize idioms. about 3 weeks ago I created idiom flashcards using index cards and page 111 in manhattan SC. Funny how it says on top that it is "unnecessary to spend time memorizing this list" but I have to admit, it's helped me out greatly! Things like range FROM x to y, when I kept having to correct myself from saying range between. native to, consider XY, declare XY, rates FOR (not of), is just not natural sounding, but they are GMAT language.
I believe that when you're down to 2 or 3 answer choices, usually doing the idiom test will eliminate any doubts on which answer choices should remain.