What my students always find it difficult while writing is to supply the correct prepositions, adjectives or adverbs that go with each other. This is actually what I find myself correcting while checking homework or writing tasks. While teaching writing, I always encourage my students to use www.ozdic.com , which is a free online collocation dictionary.
What I like most with www.ozdic.com is the sample sentences given with almost every entry. My students have vocabulary logs where they write the collocations, synonyms , antonyms, and derivatives of each word they learn. I also prepare electronic flashcards for the vocab items of every week. At first, I do not include any sentences with the flashcards which can be accessed through the computers and mobilephones of the students. I generally prefer using www.quizlet.com and www.studyblue.com to prepare the electronic flashcards. The vocabulary task for each student is to study the flashcards and copy them down into their vocab logs with sample sentences. Each entry they have on their vocab logs should have two sample sentences : one from www.ozdic.com and one they make up. On Thursdays, I assign them to write a paragraph using as many of the words they have learnt that week as possible. After I collect and check the vocab logs on Friday, I go back to the electronic flashcards I have prepared and add the original sample sentences of my students to them indicating who the sentences have been written by. My students really enjoy going online to the edited electronic flashcards to find out whose sentences have been included. While reading through the flashcards, they revise the vocab items once more again:)
Well, this is how I have been using www.ozdic.com , but I am sure you can find many other ways to encourage your students to use it.
Thanks to www.ozdic.com, collocations are no longer difficult to learn.