Honestly I doubt a book will help you write-on a journal. I would suggest getting a handle on how the BlueBook works - where the rules are, how the rule actually works, and being comfortable with using "citing by analogy" for citations that aren't listed in the BlueBook. As far as the writing is concerned, look back at your Memos that you wrote for legal writing or if you had an internship. More likely than not, you will have a fact-pattern and will be required to write a Memo giving your legal analysis on the issues.
Also, find someone that is currently on your journal that you are comfortable with. See if they will tell you what the journal is looking for in the write-on competition (i.e. do they stress BlueBooking skills or actual writing skills). Finally, utilize your legal writing prof - ask him/her what you should work on from your Memo.
Other than that, I GUESS you could look at "Plain English for Lawyers" by Richard Wydick if you would feel better about doing something else in the mean time. But I am a firm believer in practicing if you want to do any prep-work. Good luck!