Education and hands-on experience are two big keys to getting started. Education goes without saying—it's empowering to learn and understand. I took advantage of any opportunity to learn from other instructors and trainers, and, years later, journalists—shadowing, observing and then putting it into practice myself. I spent endless hours doing things for free or just getting educated to better my skills. I always say LUCK is preparation meeting opportunity!
—Chris Freytag, ACE PT, GFI, HC 

Follow Chris on Twitter at @chrisfreytag!

My best advice for breaking into the industry is don’t go at it alone—whatever that means to you, find a great network with which to surround yourself. Think about a mentor who is doing what you want to do, veterans that have been in the business for a while, people that are willing to share their missteps with you to help lead you in the right direction. Our industry operates in silos, which hampers innovation and growth. Find a great group with which to surround yourself and you can accomplish just about anything!
—Shannon Fable, ACE PT, GFI, HC

Follow Shannon on Twitter at @sfable!

It can take a while to develop a clientele as a trainer, so it’s a good idea to have another job and start training part-time. Many club companies allow managers to train clients part-time, and if being a trainer is your ultimate goal, it can be helpful to use your experience from outside of fitness by starting in a full-time management position. As you develop a clientele and improve your skills, you can make the transition to a full-time trainer. That’s how I did it when I got into the industry. I had a job in a different field, but wanted to learn about fitness so I started at the front desk part-time. From there I became assistant manager and then general manager. Along the way, I trained clients and took every opportunity to learn from other trainers and continuing education events in my area. Once I had a critical mass of clients I transitioned, into a full-time trainer role. 
—Pete McCall, ACE PT

Follow Pete on Twitter at @fitexpertpete!

Two answers:

1. If you are fairly clear on what path in fitness you want to take (trainer, group fitness instructor, hybrid trainer) and where you want to work (large gym, studio, fully independent, etc.), then get certified and get started. There’s nothing that can replace experience. And if you know the location and type of training you are most drawn to, then it is best to get out there and start doing it as soon as possible. Of course, entry-level positions will often be all that is available, but the experienced you gain can be invaluable. While this is going on, identify people who are more experienced and occupying the same spot in the industry you would like to occupy and reach out to them. Most of us in fitness have gotten where we are with some help from others and we are often willing to pay this favor forward.
2. If you are uncertain about your fitness path and where/how you’d like to work, then begin studying to get certified as a personal trainer. While you’re preparing, reach out to a variety of people in roles you find appealing and ask for a short interview for some insights. Keep the questions limited and just begin to gather some ideas for how you might be able to express the passion you have for fitness in the industry. Once you’ve done a little groundwork to create some potential options and scenarios for your career, consider some mentoring to help sort through the choices. Any full-time fitness professional that has been at it for a while has a lot of experience that has likely been learned through much trial and error. Investing in mentoring can more than pay for itself in saving you the time and headaches of making those same mistakes. Some options for mentoring include my own, Shannon Fable or Lawrence Biscontini.
—Jonathan Ross, ACE PT

Follow Jonathan on Twitter at @JonathanRossFit!

The most beneficial thing I did to break into the fitness industry was job-shadow as a high school student. Not only did I learn so much from the trainers and instructors I followed, I also got my foot in the door. From there I spent some time working the front desk, playroom and even cleaning the locker rooms. But by doing that the staff got to know me and I was able to grow as a fitness professional. Simply mailing out your résumé can work, but letting others see your true personality first is a benefit to both parties involved.
—Shana Verstegen, ACE PT

Follow Shana on Twitter at @shanamartin!