Molly has some good advise here. Actually this came up in a thread earlier this year, started by me , and ended up getting deleted due to the extreme difference in opinion on this site. Teaching Espanol in Latin America is big business exploited by many. I'm currently taking a coarse at the University and learning the basics and structure...not an easy task for someone who hasen't gone to school for 18 years, but its sticking, well, thats what my Mexicana girlfriend says anyways. If your going to learm Spannish, you may as well do it in L.America where your forced to practice everyday, this will help a lot!
I can make two sugestions: Pimsleur spannish 1 is very efective, for me anyways. It's a half hour coarse you listen to every day with your headphones. They have a way of making things clear and memorable. You can get it on E-Bay, or if your good you can download it for free through a newsgroup. I got it from my buddy this way. Do a search.
II: The city of Guanaguato, south of Zacatecas, has a very popular University. I met many Canadians and Americans studying Espanol there, all with good reviews. This is also a delightful city to stay in, with a very young vibe. Again, do a search and you'll come up with some more info.
REMEMBER!! You get what you pay for!
BTW, stay away from boardertowns in Mexico and go deep into the frontier. Boardertowns suck and are dangerous.