Hitting the gym is no sweat for Isaac Saint Felix, 21, the head trainer at L.A. Fitness in Evanston. He does it 20 times a day with clients, half of whom are students getting their ass kicked into shape. Here are Saint Felix’s top pointers to raise your heart rate and not your budget.GET GOINGTrain two to four times a week for 60 to 90 minutes, dividing the time 50-50 between cardio and weight training, Saint Felix says. And contrary to popular belief, it’s fine to get your heart pumping daily, even for a brief jaunt. “If you don’t have time for a full workout, a short run is good. It’s better than nothing,” he says. Waist-whittling miracles don’t happen overnight, but Saint Felix says you can realistically expect to see a buffer bod within three weeks. “In a week, you should at least be able to drop a pound,” he says.MIX IT UPWhen you’re up for some cardio, Saint Felix recommends the Stairmaster to accelerate weight loss, and is also a fan of the elliptical and treadmill. But not all fat-blasting machines are created equal, he says. “The stationary bike is overall the worst choice,” Saint Felix says. “It’s boring, so you don’t get as intense a workout and you’re more likely to stop.” But hopping between cardio machines, commonly called circuit training, just isn’t enough to rev you up every time. Saint Felix advises switching up your routine with hardcore group classes like boot camp or cardio kickboxing.To carve out your core, opt for pilates or yoga classes. If you’re more in the mood to go it alone, Saint Felix recommends hitting the mats with a balance-destabilizing BOSU ball for some serious ab action. “Sit-ups with a medicine ball are great,” he says. “Do three sets of those, and add a 60-second plank between each set.”CHOW DOWN CAREFULLYOn the food front, Saint Felix eschews strict rules. “No one really keeps up with a diet,” he says. He favors keeping food diaries instead. “I see too much McDonald’s and chocolate cake on students’ food diaries,” Saint Felix says. It’s all about portion control, whether on your dinner plate or at the bar. “Have fun, but cut down the number of drinks you’re having if you really want to see results,” he says.