Research. Just like brick-and-mortar schools, all distance-learning institutions are not alike. Schools such as the California Virtual Academy are accredited and have licensed teachers. This is important because most accredited colleges will not recognize an unaccredited one, and a child may end up having to get a GED to satisfy the college. All the schools listed in Resources below are nationally accredited.
Consider the costs. The California Virtual Academy is free for California residents, but if you can't send your student there because you live out of state, then you have to look elsewhere. Cost can range from free to about $2,000 per school year. Those charges are not all-inclusive because you have to pay registration fees, shipping fees (textbooks and materials) and other fees.
Explore the curriculum. Not all education is alike. Review the curriculum to make sure that your child will be taught what is required by your state, but also make sure it enforces any spiritual aspects that you want to instill in your child. Further, think about the whole curriculum and not just one aspect. Do they offer tutoring, one-on-one instruction opportunities or online activities where your student can engage with other students?
Supplement the homeschooling with local groups. If the school is great, but it is in New York and you are in Arizona, then consider supplementing the child's education with a local homeschooling group. Many get together to do science experiments, take field trips and build relationships. The important thing is that the child should not feel isolated.
Be consistent. People choose homeschooling for different reasons. It is not just people who are religious, but also military kids who constantly move, or children who are not getting the necessary help at school. Therefore, you must make sure the child is "attending" class on a consistent basis. Some schools do have requirements on attendance and have the ability to see how long a student is in "class."