Experienced public speakers know better than anyone: The first impression is the only impression and it should be the best one possible. Grabbing your audience from your opening line sets your tone and is the single most important key to success.

I'm constantly shocked by how many incredible individuals--celebrities, top athletes, best selling authors and CEOs--lose their audiences' attention in less than 30 seconds--all because of a bad opener.

I recently heard what has perhaps become one of my favorite openers of all time, from geopolitical strategist Peter Zeihan:

"My Theater Arts professor in college told us to always start out with a joke. Well... he was an ass, so I am just going to jump right into this."

The audience laughed, instantly drawn in by his irreverence for authority and acknowledgement of the potential awkwardness of public speaking. Seems a bit counter-intuitive, but trust me, it works. Peter is seriously, one of the best keynote speakers on the circuit and also the fellow who predicted Brexit - 18 months before it happened. This is a good tip from a pro - try it.

So what makes for a truly impactful opening line? How do you instantly earn (and maintain) your audience's attention?

Consider these six techniques when crafting an attention-grabbing opener to your unforgettable speech:

1. Give your audience an exclusive experience

Everyone likes to feel special. Help your audience feel they've been selected for a premier club and are gaining insights inaccessible to the common folk.

Be unique and let your audience know it's just for them. Start your speech off with an ingenious insider trick or a mind-blowing statistic from a study that hasn't yet been released to the public.

By granting your audience elite access to industry savvy, you've engaged them in a presentation seemingly built for their exclusive enjoyment.

2. Crack a joke to break the ice

Did you hear the one about the boring presentation? No, you probably didn't.

Make them laugh with that first line. The audience will like you, and they'll likely retain the actual substance of your presentation, too.

Educators know that humor helps boost information recall--so hook them with laughter, and they'll be sure to remember your line and sinker.

3. Use a prop to make your point

If you set a massive pitcher down on the podium, your audience is going to wonder why. Including a prop will not only provoke curiosity, but it can dramatically make your point.

Perhaps you are delivering a motivational speech on enduring hardship, and your pitcher represents the lemonade one should make when served lemons.

But don't just use a prop to get attention. It needs to serve a purpose--otherwise, it'll only distract from your message.

4. Make a creative comparison

If physical props aren't your style, maybe analogies are.

By making a creative comparison, you showcase your capacity for analytical thought and provide the audience with context on a subject that might otherwise be new to them.

However, just like that massive pitcher on the podium, your analogy should reinforce your point and not distract from it.

5. Involve your audience

Nothing gets an audience's attention like including them in the presentation.

Participation can be encouraged with a question, an imagined scenario, or a makeshift poll that requires audience members to raise their hands in response.

My good friend Ben Casnocha is former Chief of Staff at LinkedIn and an excellent, professional public speaker on HR. Ben gets paid "good money" to present to large audiences all over the world and he told me a few years ago, "I don't want to mail it in, I want to connect with the audience every time." Even though Ben could just stay on cruise control, he works with a speaker coach on a regular basis, ensuring he has high engagement with his audiences.

As part of his presentation style, Ben has audiences stand up, move their arms around and "vote with their bodies." For example, using body polls ("Using your fingers, hold your hands up in the air, on a scale of 1 to 10, how effective would your employees rate you as a leader - 10 being best?").

Including this kind of active engagement from the outset of your presentation will immediately grab listeners' attention, and ensure they remain on the edge of their seats for the duration of your speech.

6. Keep your opener short and snappy

"Brevity," wrote Shakespeare, "is the soul of wit."

In today's world, brevity also helps with short attention spans that are being fostered by YouTube and Snapchat.

If there is any doubt about how best to captivate your listeners, take Franklin Delano Roosevelt's advice: "be sincere, be brief, be seated."

Don't mind if I do, Mr. Roosevelt.