Q&A: Gaining experience in teaching online
Answer: That is a good question. The answer will vary a bit depending on what you teach, and on what level--especially high school vs. college. In general, though, you will make yourself a more desirable candidate for online teaching by meeting schools' needs, showing you are right for the job and making connections.
To meet schools' needs, become aware of them, then apply in ways that highlight your ability to meet those needs. Many online schools hire to fill specific needs, rather than hiring faculty members in general. You might be the most qualified math teacher in the world, but if an online school has enough math teachers already, it won't even call you back. So, review online job boards like Online Adjunct Jobs and apply for specific positions.
Next, show that you're the right person for the job. Online schools reach out to non-traditional students, and many explicitly seek people with that experience. Do you have experience teaching adult learners? The military? Mixed ages? If you don't, get some. Be creative; volunteer in order to get it, or teach an adult non-credit class. If you can add one of the numerous free classes aimed at helping classroom teachers move into the online world, such as the University of Maryland University College's modules, that's even better.
Finally, make professional and personal connections. Many schools actively seek recommendations from existing teachers about potential new faculty members. Others may not have formal recommendation structures, but put considerable unofficial weight on it. (I got my first online teaching job this way.) One of the more basic ways you can make this connection is by finding a school that offers both online and traditional classes. Start in the traditional classroom, to get your foot in the door, then transition to online learning.