Fact: Crunches can be an incredibly effective way to strengthen and sculpt your abdominals without a gym membership or single piece of equipment. But, as everyone knows, only those who do crunches correctly will reap the benefits (and avoid injury).

Sure, you've heard about keeping your rear on the floor, your chin toward your chest and your muscles engaged. But according to fitness instructor Amy Dixon, that's only part of the equation.

As Dixon explains and demonstrates in the above video, there are three all-too-common mistakes she sees when people do their crunches. Here's what you should avoid doing if you want to get a truly effective ab workout.

Mistake #1: Beginning in the wrong position

Keeping a neutral spine is essential in a crunch, but many of Dixon's clients don't have a clear understanding of what this really means and, as a result, they begin the crunch with improper form. "They end up with their lower back popping up off the floor," she says.

The easiest way get into the correct neutral-spine position, Dixon explains, is to think more about your hips than your back.

"[I] have them go to extremes: Stick their hips way out behind them, then tuck them in as much as they can. Find the spot right in the middle," Dixon says.

Mistake #2: Focusing intently on keeping your chin to your chest

Ever heard the old-school tip about pretending to hold an orange between your chin and your chest when you do crunches? Well, there's a slight problem with it, Dixon says.

"Even when you do that, it doesn't stop you from cranking with your neck," she states.

To make sure you use your abs instead, Dixon suggests an alternative approach.

"Just lift from your rib cage," she advises. "If you focus on lifting your chest towards the sky while keeping your head and neck in line with your spine, you'll only use your abs and won't have to worry about leading with your neck -- which can lead to injury and fatigue."

Mistake #3: Pulsing all day long

Doing quick repetitions of tiny crunches over and over may feel like it's giving you a real workout, but it only targets your "upper abs," Dixon says.

"You're not working all of your abs unless you explore a fuller range of motion," she says. "So, do a mix of pulses, halfway-up crunches and full-on sit ups."

Not only will this work all of your ab muscles, but it also has an added benefit.

"You'll also notice an improvement in your ease of movement on a daily basis," Dixon says.

More from HuffPost:

• What fitness instructors say vs. what they really mean
• 2 poses nearly everyone does wrong in their first yoga class
• How you're unknowingly sabotaging your arm workouts