Of course, the best way to practice storytelling is to tell stories in conversation.
But there are other great ways that you can build your storytelling skills.
Improv Theater is a great option. Improv theater can help you improve all kinds of social skills, but it's especially great for storytelling because it forces you to trust your instinct, and it gives you a great perception for what makes a good story. If you live in a major city, there is probably an improv theater class near you (and even if you're in a small town, it's worth checking.)
Toastmasters is another good option. Although Toastmasters focuses on public speaking, not storytelling, most good speeches have a few stories in them. Do a few months of Toastmasters and you'll feel much more comfortable having an audience's attention, and your storytelling skills will sharpen as well.
Finally, try writing stories! Find some sites that offer writing prompts, and try to write them. A good rule of thumb is to try to write “flash fiction”, which is a story that has 500 words or less.
Writing a story in 500 words or less is great practice for conversational storytelling, since most of your stories that you tell in conversation won't be longer than 500 words. Although flash fiction has "fiction" in the name, you don't need to write made-up stories -- it's fine to practice by writing down stories from your life.
Got writer's block? This is a great list of flash fiction prompts, and you can easily Google for more.
As you write, don't worry too much about good writing – worry about good storytelling. (You may even want to give yourself a time limit for each story so you practice telling the story, not editing it.) There's no need to try for symbolism or fancy wordplay – instead, just write a story that people would want to read.
Whew! This the end of the "Hold Their Interest" section. Hopefully by this point you're feeling much more confident in your ability to tell an interesting story.
Let's move on to the next section: building a connection. In order to keep reading, pick up my book Improve Your Social Skills on Amazon and flip to chapter 10 or sign up for a site membership - both cost just $5! If you're not ready to plunk down your cash yet, feel free to check out my learn for free section for more free guides.