I did my CFI Initial a few months ago and some things that helped:
I had a massive stack of flash cards, everything from FOIs to Aerodynamics.
Go back to the basics and be able to teach them. A DPE I was talking to who does a lot of CFI rides told me he has most people fail in the technical subject areas of the CFI PTS. So that being said, be able to teach or know most of everything in the CFI PTS, and know the Technical subject areas very well.
For most of the rides I have heard of the examiners hitting hard on: Endorsements, Regs (Part 61 in particular), Aircraft Systems, stall and spin awareness, Runway Incursion avoidance, and Visual scanning and traffic collision avoidance, and Aerodynamics.
Have a solid understanding of the Private ACS and Commercial PTS.
Also, I googled CFI Checkride Write ups and a bunch of results came up of people explaining what took place on their rides, I used that as a guide to concentrate on areas I forgot to study and it helped point out areas I needed more work on, that really helped me.
When I was studying for mine it seemed overwhelming at times, but just study hard daily and know the PTS/ACS's, FAR/AIM, PHAK, and airplane flying handbook inside and out, that way you will go into your ride very well prepared. And remember, the examiner doesn't expect you to know everything, thats simply not human, just be able to gather the necessary information and be able to present and teach it.