When someone else is riding my horse, the first thing I do is show them my first aid kit. In it, I have all the essentials and the phone numbers for me, vet, and other barn people. I also have a little chart inside that gives quick reference to the normal vital signs - this is a good thing to share with anyone that is responsible for care of my horses.
Beyond first aid, I always go over tack cleaning and maintenence procedure - just clear up the air about what you do ad don't expect (do they need to clean and wrap up bridle/saddle (reset stirrups) everytime), and if you do or don't want certain products used. I also give the option of using their own equipment - as long as it seems to fit in my opinion.
I also am direct about the fact that I don't feed treats by hand to the majority of my horses. I do allow treats in feed tub. I also specify if the person should use my treats or bring their own (I have a horse with food allergies, so they use my treats/buy the same ones).
I write down booting, procedure - both for riding and turn-out. I hate returning to find a horse missing a shoe because a bell boot was left off.
I write down blanketing instructions.
I specify grooming issues (don't trim mane with scissors, no pulling of forelock or tail, leave fetlocks natural, don't trim off facial whiskers, etc)
If my horse is being boarded, I kindly as the leaser to not incurr any expenses in my name.
I also ask that the person leasing my horse not take saddle pads, wraps, boots, etc home for any reason (including laundry) - that way my stuff doesn't go missing piece by piece.
hmmmm - sure I go over more than that - but it's late and I've been up 23 hours already today. Well, got to go!