Many airlines will offer flights to Mexico.  Many of them will also give special prices on these flights from time to time.  All you have to do is know where to look to take advantage of these deals.

First, try out the websites of a major airline.  Often, they will put out special deals for flights to Mexico.  Check the airline websites regularly to make sure you find the deals as soon as they happen, because they will sell quickly once they are posted.  You also want to get it as far ahead of time as you can.

That does not seem like the easiest way to find the cheap ticket you want, so try travel websites such as Trip Advisor or Expedia if you want to make things a bit easier.  These will give you a chart of prices and schedules that are offered by several airlines.  They will also give you their own prices if you are interested.  Then all you have to do is pick the cheapest one that works with your plans.

There is another group of websites that basically give you a search engine for airline fares.  This keeps you from having to visit a lot of different websites on your own, as these search engines automatically search all airfares on the net for you.  They usually have an option where you can reserve the flight through the same website.

Also, feel free to drop by a website called Priceline.com.  There are probably other sites like this one, but this is the most well known of its kind.  At these sites you will enter the price that you want to pay and the website will do all the searching and calling for you to get that price.  The only downfall with these sites is that the ticket is always non-refundable no matter what date or time you pick. 

Finally, there is an option on some webistes that gives you a flexible fare option.  This lets you search through prices based on the dates.  This is the best way for the actual planning of a trip.

As always, you want to plan things in advance, far in advance.  Picking the right season usually helps, as well as picking a day in the middle of the week and a time early in the day.  Stay away from weekends and night time flights and you will never get completely ripped off.  Round trip tickets to Mexico will normally be cheaper than one way tickets, as well.