1. Do not look where your eyes fall at first glance. Books at the level of your gaze are the easy-sold "best seller" books. Nothing good can come out of those.
2. Look at places where it is difficult to reach for a book (usually corners or shelves which are too low or too high). Good quality books are usually misunderstood by the sales people of philosophy books who - usually - know nothing of philosophy. But even if the sales people know their job, Murphy's Law works in this direction also, thus making the books which interest you difficult to find.
3. Select the smallest book you find there. Big books are usually by stupid authors who cannot express what they want to say eloquently or by stupid authors who - because they have nothing of value to say - delve into too many sectors without being equiped to do so. (great philosophers are of course the exception to this rule)
4. Take the book.
5. Go to the counter.
6. Pay.
7. Get receipt.
8. Go out of the bookstore.
9. READ the book in a NON-serial manner. Good philosophy books can and must be read in such a way. Do not start from the "beginning". Do not stop at the "end". Go through it as you would go through a morning breeze.