Photoshop
This first method is used in magazines, newspapers and movie posters.
They use this program to hide every single defect of their faces and bodies. The background of the photos (illumination, scenery, etc), the celebrity (body, age sings, makeup, skin, etc), all of this is manipulated in this program to celebrities look good.
Here are some sites with images of the transformations:
http://www.putsgrilo.com/arriegua/veja-o-antes-e-depois-do-photoshop-das-celebridades/
http://www.mdolla.com/2008/10/wonders-of-photoshop-celebreties-thanks.html
http://www.hemmy.net/2007/05/25/celebrities-before-and-after-photoshop/
http://www.topsocialite.com/celebrity-photoshop-before-and-afters/
Makeup
Without makeup most celebrities are different from what we are used to see. Makeup hides their age, their zits, etc.
http://www.topsocialite.com/celebrities-without-makeup/
Plastic surgeries
Plastic surgeries are very common in the world of celebrities, they want look good more and more often, even young celebrities feel the pressure and end up making plastic surgeries.
