I used to do a fair bit of English teaching when I first came to the country. First question to you is, do you have your paperwork in order so you can emit Recibos por Honorarios? If you can, the horizon really opens up for you.
But, back to your question. I think it is important to try and differentiate yourself from the rest of the teachers out there by working on a specialty, for example, English for Finance, or English for Law, English for mining, etc. Get up to speed on what you need to know (there are textbooks you can buy online) in order to teach credible, useful, high value-added English courses in a niche of high income earners, and all of a sudden you have something you can market, and an identifiable pool of people who can afford to pay for what you offer. Then, get out there and network where those people can be found. Make a simple brochure and do some mailings. Ideally, try to form group classes at companies. Failing that, try a joint venture with an English language school who might be willing to bill and help market if they get a cut of the money you bring in.
Regarding cancelations, once you run your own show, and your students recognize you as a good resource for them (because now you are also helping them with translations, and helping them prepare their presentations), you can establish your own cancelation policy which protects you. There are some very serious students out there who will respect you as a professional and will agree to these terms; you just need to find them.
Good luck, and let us know how things work out for you