I have posted several answers on Super User where my answers are the only ones posted for the questions, and they have never been accepted, nor have they been voted on. What should I do to ensure that I am receiving reputation for my answers?
I would say post better answers, but I honestly can't find any answers that I'd take offense with. I just guess you answered questions which are low visibility, and where the user hasn't bothered to test the answers.
If no answer has been accepted for a question that you answered, then poke the person who posted the question. It will help other users of the site if the person ends up accepting the solution that worked (even if it ends up not being your answer that is accepted).
Occasionally I review some of my old answers to see if they are still good answers and to find out how the person ultimately solved the problem. Sometimes I've learned something new that helps me improve my answer. Sometimes the question has been edited or clarified so my answer is not as good as I had originally hoped.
Other times it seems the person just forgot to accept an answer. In those cases, I post a comment on the question asking what they did in the end and if my solution worked for them. It might be the case that your answer solved their problem but they forgot to come back and accept it.