Lock down your phone
Your phone knows a lot about you, and most smartphone apps want access to your data. Unless they really need that information, don't give it, and don't store passwords in your phone either.
If you're downloading apps, make sure they're from a reputable source, like iTunes or Google Play. Sure, that off-brand site may look cheaper, but there could be a reason for it: The real profit is what it shifts off your phone.