Return date, reason for going
You also need to prove that you're only going abroad for the purpose you declared and that you're coming back to the Philippines on the date you stated.
A round trip ticket is a start, but anyone can buy a ticket and not use it, so you need to show you have ties to the Philippines. If you have property in the country, bring proof. If you're married, bring your marriage certificate. If you have a child, bring his/her birth certificate.
Prepare an itinerary and know it by heart. Know where you're staying, where you're going, and when you're coming back. Bring the receipts of your plane tickets, accommodations, and any pre-booked attractions.
Some people travel to one country, supposedly for the purpose of tourism, and from there they go to another country to look for a job. And that's really understandable – after all, we're all just trying our best to provide for ourselves and our families.
But it's also the IOs' job to prevent, as much as possible, things like human trafficking and drug trafficking. They don't have a lie detector, so they need to rely on their instinct and experience. And they do have a lot of experience with all the ways people have tried and will try to bypass the system.
Speaking of which, never lie to the IO.
Just don't. Lies have a way of coming back and biting you in the behind. What if you tell the IO that you're, say, an engineer and he asks for a PRC ID? What if you tell the IO you're still a student and he asks for your ID and proof of enrollment? What if you tell the IO your sole purpose is tourism, but he searches your baggage and finds tons of application letters and CVs?
I don't know if the Bureau of Immigration keeps a record of all the things you tell them, but lying will be a black mark against you for sure. Some people are good at lying and get away with it, but it's just not a good idea.
It's better to over-prepare than to under-prepare.
Some people breeze through Immigration with just the basic requirements and only a few questions. Some have a harder time. Many actually get offloaded. While there are many things we can't control – like the IO's mood that day, or the fact that we don't look rich enough to be traveling abroad (yes, profiling happens) – we can minimize our chances of getting offloaded by preparing anything and everything that may be required from us.
Be especially prepared if you're young, female, single, or a recent graduate. If it’s your first time traveling abroad, if you have a history of being offloaded, or if someone else is paying for your trip, make sure you have all the necessary documents on hand.
On my last travel, I prepared probably 50 pages worth of documents (plus photocopies stashed in my check-in luggage). Only two documents ended up being seen by the IO and I was only asked two questions – but that's better than if the IO had asked for documents that I failed to prepare (even though I could have) and I got offloaded as a result.
Dress decently, speak confidently.
Don't overdress or underdress; wear clothes that say you're a respectable traveler. Answer the IO's questions in a respectful but confident manner. Again, don't lie, but don't give out unnecessary information either. Just get to the point and give consistent answers.
Even if it feels like you're already being interrogated, don't panic. If you've prepared all the documents you are likely to need, and if your intentions are truly honest, you've done all you can do – draw your confidence from that.
All this might seem like an unnecessary hassle, but remember that being offloaded is not the worst thing can happen to a traveler. Human trafficking, abusive employers, being stranded or even jailed in a foreign country with no money and no one to help you -- those are much worse. Our right to travel is guaranteed by the Philippine constitution, but until the screening system improves, we might as well just do everything that we can to avoid being offloaded.