Each product we feature has been independently selected and reviewed by our editorial team. If you make a purchase using the links included, we may earn commission.
Ariana Grande and Rihanna
Credit: Larry Busacca/Getty Images, Jon Kopaloff/FilmMagic

Just because it's soon time to start wearing more fall layers doesn't mean it's time to let go of that summer bikini body you worked so hard for. The good news: You're already motivated! The straight talk: You're going to have to work for it, just like Rihanna, Ariana Grande, and Alicia Keys, who are just a few of super-trainer Harley Pasternak's star clients.

Harley Pasternak - Lead
Credit: Angela Weiss/Getty Images

We caught up with the star trainer inside his Los Angeles studio for a sweat-drenching workout hosted by Fitbit and asked him for tips on staying in swimsuit shape all year round. "Everyone is motivated. Not one human being I’ve ever met doesn’t want to look or feel better," says Pasternak. “But there's this idea that we have to be able to be in our spin class or go to our gym in order to be healthy and stay in shape. So if we don't have time to go to the gym, we don't do anything. It's that way of thinking is what gets in the way of motivation and action. That’s it."

Harley Pasternak - Embed 3
Credit: Angela Weiss/Getty Images

So if we're already motivated, what's the key to staying in shape all year round? Set simple goals and stick to them, says Pasternak. "If you say, 'I’m gonna be a raw vegan and run a marathon and do CrossFit,' you’re dooming yourself to fail. Instead say, 'I'm going to walk an extra thousand steps a day, I’m gonna eat a bit more fiber, and I’m gonna turn my cell phone off when I go to bed and get 20 more minutes of sleep a night.'"

Harley Pasternak - Embed 2
Credit: Angela Weiss/Getty Images

Pasternak also prescribes a simple yet effective daily exercise combo of cardio—whether it's racking up 10,000 steps on your Fitbit or hitting a group barre class—and a single exercise move to sculpt, tone and tighten. "For each day of the week, pick one resistance exercise that works a different body part," he says. "For example, do lunges or reverse lunges every Monday, no matter what. You don’t have to change into a [workout] outfit. You don’t have to go to a gym. Just do four sets of 20 reps every Monday, no matter where you are."

Harley Pasternak - Embed 4
Credit: Angela Weiss/Getty Images

As simple as Pasternak's tips are, the trainer says they only work if you commit to putting in the work every day and making sure that you're always challenging yourself, no matter the season. "It's great just to get moving, but at a certain point you plateau. That's when you have to say, 'Ok, it's time to take things to the next level." So what does a star trainer feed his body? Pasternack says he enjoys snacking on Siggi’s yogurt, Smartwater, and "The White Smoothie," a breakfast fix from his own book, The Body Reset Diet ($15, amazon.com), that includes apples, bananas, almonds, plain yogurt, and cinnamon.

RELATED VIDEO: How to Get a Body Like Gwyneth Paltrow