Here's the 411:
Mercury rules over the mind's processes, studying, communication, businesses, travels and the like. When Mercury reverses its direction, all these areas are affected as well.
The mind turns naturally inwards and people tend to analyze more the own thoughts and follow the common thinking patterns, rather then be curious and eager of new intellectual experiences or challenges. This helps the meditation or the thorough lonely long-term study of a specific matter, but it affects the study of new subjects, the communication with the others, the attention oriented outwards.
Businesses, travels and communications tend to experience delays and different problems. Computers and other processes that work with information may experience crashes, unexpected failures.
Don't enroll to courses, don't buy expensive Mercurian items (books, cars, mobile phones etc.), don't sign important contracts and do not marry.
What is this Mercury retrograde period good for?
It is definitely a very good period for some actions. No time is completely bad for anything, there is a reason in everything happens.
The key is the reversed direction of movement: take any known Mercurian action, reverse its flow, consider the keywords "re-doing something", "double-checking", "finish the old projects" and there you are, you've found the good side of Mercury retrograde.
For instance, you may want to read again a book you particularly liked, a subject you studied before, meet and discuss with old friends you haven't met for a long time, travel to places you've already been to before.
This is an excellent time to work on old projects that never got to be finished. So, think about the things you started and never finalized.
Next, you might wish to prevent any bad things to happen to you: so double-check your agenda, call your business partners to confirm that everything goes as planned, have everything ready before the deadline and leave some extra time for unexpected events. Make copies of your important files and documents, save your work more often.
The other solution is to go on vacation or at least slow down the pace of your projects. You will find that going slowly during the Mercury retrograde period will spare you many efforts of redoing the same action that wasn't performed right the first time.