Like a lot of personal trainers, Gavin McKay marvels that so many people still think the road to great abs is paved with hours at the gym.

"Literally still, still today, after all of the information that's been out there about crunches not getting you abs, they still think that's somehow going to do it. It's absolutely insane," said the founder of Unite Fitness.

Strength training helps build muscle, but you can't see it if it's shrouded in fat. If you want to slim your belly, you'll need to lose body fat - but how low should you go?

McKay says men who are really serious about seeing their six-packs should aim for around 15 percent body fat; for women the figure is closer to 20 percent.

Start in the kitchen

A diet built on low-carb, low-fat, and low-inflammatory foods is how McKay keeps his six-pack. Inflammatory foods, like processed sugar, processed wheat, and alcohol, put the body in a fat-storing mode, rather than a fat-burning mode.

"It all boils down to not eating the cookies and chips and drinking beer. It's called a beer belly for a reason," McKay said.

Here's how important diet is: "You don't need any exercise to have a flat stomach and abs if you're approaching it with the right diet," he said.

Then hit the gym

But a combination of strength training and cardio will help you on your way, in addition to providing all the other benefits of fitness. Forget hanging out and chatting between sets. Strive to keep your heart rate in the 70 percent to 90 percent max zone while splitting your workout evenly between strength training and cardio. McKay suggests at least 30 minutes of cardio and 30 minutes of strength training.

"I don't recommend just doing one thing. I would not recommend running every single day or even running for an hour four times a week; that will just lead to injuries," he said.

Keep at it

Along with diet and exercise, the third key is consistency. Expect to start seeing results after about three months of solid effort, with your biggest visible changes taking place between three and six months.

Still can't let go of those crunches? McKay advised doing counterbalancing exercise as well, like squats or dead lifts.

If all you do are crunches, he said, "you're strengthening, but you're also tightening. When you're squatting, you're opening up. When you're dead-lifting, you're opening up. So you need to really balance that out with opposite movements."

Read more Sports Doc for Sports Medicine and Fitness.