Story highlights Fitness trainer David Kirsch started working with Kate Upton in August

Upton did a strict cardio and sculpting boot camp, Kirsch says

Bikini model also followed Kirsch's ABCs: no alcohol, bread or processed carbs

OK, we can admit it. Even though Kate Upton spent six days in Antarctica posing in minimal clothing in below-freezing temperatures -- which she says led to hypothermia and frostbite -- we're jealous.

Why? Um, because we don't look like that in a bikini.

Upton is the first model to be picked to do back-to-back covers for Sports Illustrated's swimsuit edition since Tyra Banks appeared for the second time in 1997. And you only have to look at the photos to figure out why.

Fitness trainer David Kirsch started working with Upton in August to prepare for the December photo shoot. We reached out to ask him about her exercise regimen, the biggest mistakes he sees clients make at the gym and why a strong mind is just as important as a strong body.

CNN: What was Upton's training schedule like?

David Kirsch: The idea was to step up the intensity and duration of the workouts, making it a strict cardio and sculpting boot camp that entailed body weights and resistance bands, sprinting and calisthenics, shadow- and kickboxing.

David Kirsch is a wellness expert and celebrity fitness trainer.

At the beginning, we worked out seven days a week, twice a day. Then we brought it down to five to six days a week for one hour a day.

CNN: Was there a specific area she needed or wanted to tone?

Kirsch: Kate has beautiful, feminine curves, and I didn't want her to lose a crazy amount of weight. The goal was to lengthen and tone her thighs and inner thighs, and keep her butt perky.

It wasn't about dramatically changing her body; I didn't want to do that. Just tighten, tone, lengthen.

CNN: What was the most difficult thing for Upton? What was the easiest?

Kirsch: Most difficult for her was the sprinting -- getting up to a sprint was challenging. And the easiest was the boxing and kickboxing, mostly because she loved kicking my ass!

CNN: Do you have a training motto or No. 1 rule for your clients?

Kirsch: My training philosophy is sound mind, sound body. That is my No. 1 rule. That and follow my ABCs: no alcohol, bread or processed carbs.

CNN: What is the biggest obstacle people face in getting to the gym?

Kirsch: I think the biggest obstacle for most people is that they don't think they have the time to work out. But if you follow my advice and do my express workouts, or break your exercise into 10- to 15-minute increments throughout the day, you'll find that you have plenty of time to exercise in small doses.

CNN: What is the most common mistake you see people make while working out?

Kirsch: They don't subscribe to my mind-body philosophy of training; they're not doing the correct exercise for their body type, or they're distracted, socializing, using PDAs instead of focusing and visualizing.

CNN: Give our readers the ultimate piece of advice -- something you've learned over the years that they might not know.

Kirsch: The mind is more powerful than any muscle in our body. With our mind focused, we can will ourselves to unbelievable greatness.