Reading is the best way to learn word choice.

L2 learners can have grammar stuffed down their throats endlessly, and after a while they just lose interest in developing skills. There are so many ticky-tacky little rules about which preposition to use and where in the sentence, and it seems to impossible to teach a point of grammar in just a few minutes if their regular instructor hasn't been able to teach that same point in the classroom. After a while, it becomes not so important to the learner to use all the grammatical crap in English properly.

What do writers in any genre do to better their own writing abilities? They read. So apply that to helping the students in your writing center, by encouraging them to read as much English at their level or just slightly higher. They'll see different grammatical structures they are currently struggling to learn and develop a better understanding of how they are used. If they are struggling with a particular passage or sentence in their own writing, they will have examples in what they've read recently.

English grammar is changing, and changing in a hurry, thanks to the cyberage. Cyberlanguage has simplified some grammatical structures, to the point of often not having the same meaning to different people. Often, ESL learners expose themselves to cyberlanguage more than standard English, and it becomes a problem. It seems to them that if its OK to use it on the net, why not in daily life? The problem is that they develop bad habits first, which need to be broken later if they want to excel academically or professionally in an English language environment. To try to counter that, try to encourage book and article reading in an area they are interested in pursuing.

The only other little trick is have them buy a little note book they can call their grammar note book. Whenever you need to correct or explain a grammar point, have them write it in their little notebook in the correct sentence form for refering to later. Just a little pocket sized book is big enough. You can refer to it later and remind them how they used "to" or "of" or whatever.

And always remind them that we all have trouble sometimes trying to find he right way to say or write something, and need to rewrite a sentence or passage several times until we are satisfied. That might ease their frustration level some.

Good luck!