How do they make actors/actresses look so good?

By Alan Chenkin, Learned about celebrities by reading the tabloids in the checkout aisle.
From Makeup to Camera angle, making actors look good/younger/better is the magic of the makeup artist and the director.  
Movie stars are frequently caught without makeup (by naughty paparazzi, clearly without makeup and any primping).  A movie star’s look is the magic of Hollywood.  FYI, the Paparazzi will try and catch a movie star unprepared and enjoying his/her right not to wear makeup, etc., and the tabloids may have even photoshopped the photos to make a movie star look even more disheveled.  

 Marilyn Monroe can even make a potato sack look Good! – Amazon

You can see that makeup artists enhance a person’s look and refine it for the camera, erasing blemishes, highlighting eyes and cheekbones, and  making  their appearance alluring for the camera.  I hope my comments and references give you the answer you seek!
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Edited and expanded from my answer on Quora