When I was in year 9/10, I used to do a lot of reading. Whenever I found a word I did not understand, I would go and look it up in the dictionary. If for some reason, I thought that word might be useful, I would get a small piece of paper (about the size of a post-it sticky note) and write down the word. On the other side of that piece of paper, i would write down the definition of the word with an example of how the word is used (eg: a sentence a two with the word in it). I would then keep all these little pieces of paper in a box and whenever I felt like varying my vocab in an essay or writing task, I would take that box out and look for words that might be useful. This really helped me with my writing then.
Now Im too slack with writing and just write anything. LOL. If my essay sounds boring, I just go to MS Word's thesaurus thingy and yeah...LOL