Well, apart from taking malaria tablets (which I didn't bother with on my last visit) and not drinking unbottled water, I haven't worried about taking any special precautions.
I've cleaned my teeth in tap water, I've taken ice in drinks and halohalo, I've eaten in some seedy-looking local eateries, I've spent lots of time out at Ruby's family home in the middle of rice fields and plantations (water supply from a well), and the only time I've had any sickness was after eating the seafood platter at one of the more respectable eateries (various shellfish, squid, etc).
If you get to the Davao area, give us a call .. I'll pm my Phils number if you wish.