In your public speaking you might say, "Getting this contract signed is as impossible as trying to smuggle daybreak past a rooster." Contracts and roosters don't have much in common (which is funny), but in this case, the presenter is telling you what they do have in common. Getting the contract signed and smuggling daybreak past a rooster are both impossible. 

You could shorten the last simile by changing "as impossible as" to "like." 

Getting this contract signed is like trying to smuggle daybreak past a rooster. 

In this case, the audience must make the interpretation that both are impossible. It's good to make the audience think sometimes because it forces them to be involved in the public speaking program. 

A recurring theme with me is that humor surrounds you wherever you go. I got a great simile out of a child's joke book I acquired (if something is valuable you acquire it) for 10 cents at a flea market. I now use this line in public speaking presentations all over the country. I do a public speaking seminar called Business Lite: Low Cost/No Cost Ways to Improve Productivity. In that seminar I talk about how employees feel at work. I say, "Sometimes you go to work and you feel like a turtle with claustrophobia; you've got to be there, but you feel closed in."

 I like to mix and match many types of humor in one concise chunk for my public speaking. Here's a simile that I just love.

If you put his brain on a matchstick, it would be like rolling a BB down a four-lane highway.

Let's break this one-liner down to see how several different forms of humor were used. Putting a person's brain on a matchstick and rolling a BB down a four-lane highway are both ludicrous juxtapositions. No one is going to put someone's brain on a matchstick, or roll a BB down a four-lane highway. This piece of humor is a simile because the two ludicrous juxtapositions are compared with the word like. 

The effect of the simile is to exaggerate how small this man's brain is. So, three different types of humor juxtaposition, simile and exaggeration were combined to make a great one-liner. These are the types of relationships you would explore if you were feeling adventurous and decided to write some of your own humor for your public speaking. Many of the one-liners you will encounter will be combinations like this. You don't have to be able to dissect them like I just did. All you have to be able to do is pick the ones that make your point (in this case similes), and use them where and when appropriate.