a hard checkride
The CFI initial checkride is, IMHO, the hardest one out there. Granted, I did my initial back in 1992, but my oral exam was a little over 4 hours and the flight was an hour. Know how to draw up a lesson plan, and be able to recite definitions verbatim from an FAA source. (I was asked to define Mean Aerodynamic Chord). Be ready to show how a weight change/shift will affect CG, answer weather chart questions, MMEL and MEL questions, and as previously stated, make sure you have all YOUR paperwork in order (8710, endorsements, etc), If you are sloppy and unorganized, the examiner will believe this will reflect your students' performance as well.
Get the AC for CFI endorsements and hang on to it.
Don't BS if you don't know, but know where to find the answer.
On the flight, talk, talk, talk. I was told by my instructor that I only had to "teach" the one maneuver that was selected by the examiner, and the rest of the manuevers would be just to show commercial pilot proficiency . However, when we got in the airplane, the examiner told me, "you had better start talking to me if you expect to pass this checkride".
Remember, as the PIC you are in charge. Don't let the examiner intimidate or distract you, as some will try to do. I had to tell mine to pipe down as he purposely kept on talking over ATC as they would try to give me instructions.
Best of luck.
Hvy