One on the most important things to remember is when they cannot remember who you are or they say things that you know they would not have said before hand ....this is the disease speaking and you must not take it to heart. This can be very, very hard.
One way to 'cope' is when you walk out the door think of something amusing they have said and have a laugh. You are not being disrespectful but using this a way of coping.
It can be very sad watching a loved one change before your eyes.
There is no handbook for this disease. It can progress very quickly or take its time. Also the person can get very stressed as they know they are not remembering things that they should.