Remember you've just paid good money for this book.
Place your new book on a table or desk. Open it enough to get a few fingers between the cover and grab book block. Let the covers fall by themselves. Then slide your fingers between the last few pages of the book block. Press pages and cover down gently. Do the same on the front. Keep doing this front and back until you reach the center of the book.
Depending on the style of book the covers pages will either fall down easily
or will need a bit more of a push.
DON"T crack open the book and press down to a flat position. That's where the term 'cracking' a book open comes from. You could literally crack the spine open.
This is why, especially with cheap paperbacks or other glue books, the spines fall apart.
You could also start a tear in the paper of the text block.