Not knowing what you've used to prepare thus far, definitely pick up the NCEES practice exam and Joseph Schuster's book of practice exams. In my experience, the NCEES practice exam was a bit easier than the real thing, but Mr. Schuster's practice exams were closer in degree of difficulty.
Beyond these two sources, there isn't much practice material available for the constructed response problems. I ended up working these same problems over and over until I understood every aspect of the solutions inside-out. You might also want to see if you can obtain older copies of these books (based on older versions of the codes) just to have additional problems to work.
Best of success to you!