Big Don't for me is:
Do not read too far in advance for yourself or others, especially when in say 6 years you know a difficult transit is coming up, if you read too far ahead this may totally change how you react over the next five years it may stop you learning some of the important life changing experiences you need to go through in order to cope with those difficulties coming your way, if you do a reading for someone this far ahead I think the best thing you can is advise on how to make sure they learn some helpful coping mechanism for when they do have to cope with that transit.
For instance if you saw someone suffering with a difficult illness that could be life threatening in the future, tell them that it would be advisable for them to make some life changes to increase their health, and get out and enjoy exercising, to work on their body and soul and enjoy the company of their friends. So when that difficult transit comes along they will perhaps be able to overcome the health issues easier, will have a good network of friends to help them through it.
Or if you saw someone was going to have problems with employment or relationships, get them to work hard on their self confidence and personal security, so if that again they can cope better and are not as damaged when these things occur.
I myself began to read too far ahead in my chart and saw so many problems ahead in the coming years, I got depressed despondent it was almost a self fulfilling prophecy that I would suffer depression and this would effect my relationship. Somethings need to be held back until that person has reached a level where they are ready to cope with their problems and are adequetly equipped to learn from them but not irrecoverably damaged as a soul.
This is something I've personally gone through recently and think is one of the most important of the do's and don'ts of astrology, I think some people get off on the whole power thing of being an astrologer, like they have secret knowledge and they can change someones life, play god.
I personally have always seen astrology as a way of counseling people acting as a sort of life couch come therapist. Maybe that's because I have Aquarius/Pisces in the first house, I like to give people the freedom to develop themselves make any necessary changes to achieve their life goals and to support them with those changes they are finding challenging.