Guest speakers can be such a powerful tool to your classroom and are hugely underutilized. I don't think I truly understood their value until I started teaching project based learning. Part of PBL includes having a public audience (you can check out a blog post on this topic here). As I developed projects and started bringing in people from the community it made a huge impact on my students.
Why are they so powerful? First, students are used to hearing us teach every day, and don't always give us 100% of their attention (who am I kidding, they RARELY give us 100% of their attention). But whenever I've had a guest speaker come in, the students seem to hang on to their every word. Another reason they are invaluable is because they can bring in a level of specialized content knowledge that you don't have.
For example, I recently had my students complete a project where they had to design a food truck. We had been learning about sustainable agriculture, macromolecules, and nutrition. Students were asking questions I didn't have specific answers for, like how much local ingredients would cost and how they could decrease their company's carbon footprint. I could have done some internet research to help them find the answers, but why not go straight to the source? I sent a quick email to the owner of a farm not too far from our school asking if students could ask her some questions over the phone about her business. She was more than happy to speak to them and talk about her organic farm and the struggles of starting a small business. They were able to record the phone conversation and refer back to it later as they prepared for their presentations.
Where to find guest speakers
I promise when you begin to reach out to people in the community, you will be surprised how willing they are to come in and speak to your students. You won't always get a yes, and you won't always find people that can stay all day and speak to multiple class periods. (One way to solve the multiple class periods issue is to record the presentation and show it to your other classes). I've had luck tracking down people willing to speak to my students from almost all of the places listed below:
The Beauty of Modern Technology
While it is always ideal to have someone come in and meet with your students personally, this isn't always possible. But there are other options! Websites such as www.skypeascientist.com allow you to do a skype or google hangout session with a scientist. You choose which type of scientist you would like to skype with based on what you are teaching and they will match you up accordingly.
I've also had students do phone interviews with multiple people ranging from professors at our local university to food truck owners. If you email people and ask if they have 10 minutes to spare for a quick phone conversation they will almost always say yes. As a bonus, it is good practice for students to learn how to speak professionally on the phone.
A closing tip....
Most guest speakers that come in aren't used to speaking to teens. They do a great job, but don't have the same practice you do. One thing I've noticed is that they aren't used to what us teachers call "wait time." They tend to ask a question to students, wait about 2 seconds, and then answer it if they don't see hands go up.
When the guest speaker comes, pull them aside quickly and give them a gentle tip to wait a while after asking students a question. Explain to them that students need much longer processing time than adults. I've never had anyone be offended by me giving them this tip, and it's made the classroom discussions much better.
Good luck finding a guest speaker and enjoying a day of listening and learning instead of teaching!