Here's my unsolicited advice:
-Actually learn Spanish. Learning another language yourself will give you some personal experience to go on. You'll have a better grasp of what your students need. Also, you'll understand why students make the mistakes they do and be able to give them better feedback.
-The only thing ESL certifications do are sit pretty on your resume. Get thyself to something useful, like a linguistics class. I'd recommend taking a hispanic linguistics course, a phonetics class, or an English language course (like the kind they make editing students take) in college.
-Learn how to spell words in English. Standing in front of a class trying to remember if the 'i' comes before the 'e' in 'receive' is one of those things that spellcheck doesn't fix.
-Share your culture ... via stories, family photos, etc. We've done a great job of exporting our pop culture to the rest of the world... but students are not usually familiar with everyday American life. My students are always riveted whenever I talk about our house (they are amazed that it has a yard!), my family, what my parents do, etc.
-Teach like we teach in the US. Students will flip when you let them do things like write on the board, perform skits, draw pictures, listen to music, etc. In Spain, most subjects are taught through lectures. Here, that whole 'multiple learning styles' thing is only for preschoolers.
I came to Spain to go to bookbinding school, not teach English. But I'm glad that I've made the time for it. Teaching English for a few hours a week has actually improved my Spanish and gets me to ponder language in a way that bookbinding school does not. Now I feel like I see my own language from a different perspective.