Have some money ready for the airport tax. At last check, it was 500 pesos. Gets higher every year, apparently.
At the document check, you're asked to fill out a "departure" card form. I'd assume they don't really check what you wrote, but I'd hate to see what kind of hassle you'd get into if they caught some foolishly written, false info.
Though it really looks nice in the Duty Free airport lobby, I'd advise against buying clothes there, given that the prices seem pretty high. Colognes and perfumes are priced decently though. Electronic goods are also reasonably priced, but, imagine the hassle if something was wrong with the product! How do you return it for a refund?
Food and drink at the 'port are massively
expensive (duh, so what else is new? Those things cost a lot at pratically
any airport in the world). So, if possible, it would be a great idea to
bring some food along. A salad is good in this
situation, as is a cold sandwich or two. Drinks are another thing, probably a bit harder to bring along and you'd probably rather save it for the flight if you did bring something. There are filtered water fountains, and if soft drinks are a necessity, those can be had at lesser of an outrageous price than with the food items.
Snacks are surprisingly inexpensive at the duty free "deli", with things such as candies, peanuts, almonds, walnuts, and others. Good idea to buy some for consumption during your flight, seeing as how the prices are pretty good.
Avoid purchasing magazines at the airport! Massively overpriced! Buy them somewhere other than the airport if possible.
Expect anything from a 15-120 minute delay on departure. They give the darndest reasons, too.