OK! EXCLUSIVE: Jen Garner's Relationship Advice for Gal Pal Jessica Biel

Jennifer Garner is helping Jessica Biel and Justin Timberlake get their romance back on track. When Jessica told Jen she was having problems getting Justin to commit, "Jen advised her to back off and stop being so clingy," a source tells OK!.

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The Valentine's Day co-stars have become fast friends in recent months.

"She even came up with a 'relationship cheat sheet,' including ways Jessica could spice things up and play hard to get at the same time," the source says of Jen's advice to Jessica.

Justin and Jessica have endured a stormy relationship — one that he has come close to ending several times.

"Jen told Jess to calm things down, and remind Justin what attracted her to him in the first place."

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Among Jen's Tips:

• Play hard to get by being less available.

• Limit phone calls when they're apart.

• Stop quizzing him about other women.

• Give each other sexy massages.

• Play nice with his guy friends and his family.

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It seems the advice is working!

"Jessica and Justin are in a much better place," says the source.