Normally we wait for the captain to make that announcement, then we proceed to ready the cabin. On rare occasions, the captain will phone the purser and advise her/him to prepare the cabin and be seated (for many different reasons; weather being the main one). The reason he doesn't usually announce that is that once the flight attendants are seated, he doesn't want them getting back up. Now if he says "Prepare" too early, passengers may start getting nervous or will get up, walk around or go looking for the toilets.