For an MEI, I'd try to get a Seminole POH and study the systems and procedures in depth. Also be prepared to give a lesson plan on a multiengine topic such as a Vmc demo.
For the CFII training, be extremely proficient in approach procedures and cockpit management. If you have never flown a GPS approach, spend time now learning about them. Garmin has a PC simulator for the GNS430 that will show you every feature of the unit, and will save you a lot of frustration if you can use its basic features when you arrive.
A book that I highly recommend is "The Complete Multiengine Pilot" by Bob Gardner. I read it before coming to ATP, and find that my students who have read it before arriving breeze through the oral exam.
I have compiled a 31-page packet of ground school info, checklists, and forms for arriving students that complements the supplement. It's too big to email however.
Also, after your first flight try and get some "cockpit time" in the airplane, just running through checklists and getting familiar with the locations of the various switches and controls.