If you don't want to have a candid camera picture of yourself sold to you for an outlandish price by airport photographers, cover your face when you start seeing guys with cameras.
Be prepared to wait a good 30-180 minutes for your luggage to arrive in the baggage carousel. It takes THAT long!!!
Seeing as how it takes so long to wait for your luggage, a snack is something good to bring along, because there's no food vendors in the luggage or customs areas!
Make sure you have a relative or otherwise trusted person to pick you up. I hear stories all the time about tourists getting bilked by taxi drivers at the airport so, watch out for that! You'll have to find your pick-up party/person at the so-called "welcoming area".
If no friend/relative is available, and you need to get somewhere, the airline or airport will be happy to help you get a registered cab driver who will be obliged to serve you the right way. They can be found just past the customs door, and before reaching the welcoming area.
If ever possible, reducing the amount of luggage you bring will make things markedly easier for you.
Dress light! It can be a real steambath in MLA, especially when you leave the cozy air-con-fines of the 'port. A water bottle can go a long way in making you feel better too, so you'd better bring one or two of those.
If you need to get duty free items, get them while on the plane. Sometimes if you try to get items at the 'port, they jack up the prices noticeably. But, if you can spare the time to go to Duty Free Phils. (across the street from the 'port), the prices are comparable to what you'd pay for while in the air, but a bit more of a hassle. How do you go shopping with all your luggage in tow??? Keep your travel documents and stuff handy, they'll ask to see them for verification that you're eligible to purchase items duty free. You'll have 48 hours from your time of arrival in the Philippines to make use of your duty free priviledge.