Consider getting further training through a graduate school program. Read more in the Idealist.org Education Degree Overview.
Teacher education programs are changing rapidly, so it is wise to talk to a lot of people (teachers, administrators, grad schools) and investigate your options widely. If the idea of being in school doesn't appeal to you, there are an increasing number of graduate programs that emphasize hands on teaching.
Many universities offer graduate programs leading to elementary and secondary certification. Peterson's on-line guide to higher education institutions lists education graduate degree programs in 37 areas from over 1,000 U.S. institutions. Areas of specialization include topics such as Bilingual and ESL Teacher Education. There are many other on-line guides that you can use to identify potential programs.
While many great programs are available to pursue teacher certification, no national system for evaluating them. Currently there are two national organizations working on accreditation standards for teaching programs: the National Council for Accreditation of Teacher Education (NCATE) and the the Teacher Education Accreditation Council.
See Hands-on Classroom Teaching Programs for some initiatives that combine classroom teaching and graduate work, and Teaching at a Public School without Certification for programs that provide alternative routes to certification.