Mexico
Qualifications
Immigration and employment requirements are not consistent across Mexico. Officially, immigration requires a degree relevant to education or English. However, immigration offices in differing states around the country often cede this authority to the employing school or university and will issue an FM3 working visa based on little more than the employer's documentation and an invitation to the foreign teacher. 
Teaching employment varies from language schools, to private elementary and secondary schools, to universities, to language consulting firms with a focus on companies. Qualifications also vary from having a TEFL or CELTA certificate, to holding a degree from any field, to holding a Masters in Education with plenty of teaching experience.
Types of English teaching jobs
For those new to teaching, a language school job is the most common entry point. Language schools can be found in almost every city, from a small local English school to a large national or international chain school. Those with experience will find employment opportunities in private elementary or secondary schools, as well as universities with English departments or an affiliated language school.
Salary
Salaries are not high by world standards, and vary throughout the country. Language schools often pay hourly wages, though many pay a monthly salary. The maximum one might earn at a language school is $1,000 US/month - very good locally, but difficult to save and send money home.
Private schools and universities offer better compensation, ranging from $600-3000 US/month, often with benefits such as accommodation, health coverage, and vales (coupons for local purchases, most often for foodstuffs).
Working conditions
Split-shift work is common at language schools, particularly those that cater to companies. Early morning and late afternoon hours, with a large gap or 'siesta' in the middle of the day, are also quite common. One rarely expects to work a full 40 hours/week at a language school, though it does happen. The average teaching load at a language school is about 25 hours/week.
Private school and university positions most often entail a minimum of 20 teaching hours/week and a further 10-20 hours of grading, lesson preparation, and administrative work. Resources are often limited. While a teacher is rarely expected to bear out-of-pocket expenses, schools are not very generous with budgets for computers, internet, and high-end equipment.
General information
Many people look to teach in Mexico in resort areas on the assumption that there must be a great need for English in these places. The reality is that people employed in these areas generally already speak fairly good English, as a condition of their job, so there is not nearly as much employment available for foreign teachers there. In fact, due to high competition for teaching employment in these areas, wages are depressed, yet cost-of-living is high because of the large influx of foreign tourists.
The largest cities, such as Mexico City, Guadalajara, and Monterrey offer the most opportunities for employment and the highest wages, while cost-of-living is much lower than in coastal areas. Smaller cities also offer many opportunities. The northern desert area is sparsely populated, and the most southern states (Oaxaca, Guerrero, Chiapas) are low development areas. The best teaching opportunities are in the central states.
Safety and Security
Mexico has an undeserved reputation for being unsafe, though precautions are necessary. As in any developing country, a foreigner may be the target of many stares and invitations. Crime in Mexico is rarely violent and mostly restricted to pickpockets at transportation hubs. Foreign women will find themselves the object of many males' attentions, but rarely in a dangerous position.
Many areas of the central states of Mexico are developed to first-world standards - a surprise to many. These areas are generally safest, including the cities of Monterrey, Mexico City, Guadalajara, Cuernavaca, Queretaro, and Aguascalientes.
Links
http://www.innovative-english.com - Teachers Latin America, information on TEFL training and teaching
http://www.countryreports.org/country.aspx?countryid=160&countryName=Mexico - Mexico statistics
http://www.mexonline.com
http://www.teachers-international.com/wordpress/ - Interesting teacher's Blog
http://www.eslcafe.com/forums/job/viewforum.php?f=26 - Mexico teaching forum
http://www.teflwatch.org/forum/index.php?board=4.0
