Like regionalcap said above, if you've let your instructor "currency" lapse by not getting a renewal every two years, your only option is another checkride with an examiner. That said, you could take the Jeppesen/AOPA/Air Safety Foundation online refresher as a study aid (I renewed my CFII that way last time, it was an OK program) I haven't tried it, but I hear the Gleim version is good too. American Flyers supposedly has a one-time sign-up fee that provides "lifetime" access to their refresher program, but I can't speak to the quality, never tried it.