The ENQUIRER exclusively reveals Jennifer Aniston, newly engaged to much-younger John Mayer, spends a small fortune each month on lavish beauty treatments.

Like most actresses, the 39-year-old former "Friends" star has always worked hard to keep up her looks, but since she started dating 30-year-old John Mayer, she began spending huge amounts – about $20,000 a month – on her appearance, says a source close to her.

"Jen is doing whatever it takes to hold back time – short of plastic surgery," a friend told The ENQUIRER.

Her monthly regimen includes private instruction with top yoga guru Mandy Ingber, and personal training sessions, both done three times a week – at a cost of about $3,000, says an insider.

Jen also pays a private chef $3,500 a month for organic, high-protein, low-fat meals.

"Jen is insecure about her legs, so she goes for anti-cellulite spa sculpting treatments twice a month," revealed the friend.

"They cost $1,000 a session, but Jen swears that the sculpting keeps her skin supple and smooth.

 "She calls the treatments ‘necessary tune-ups."

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