how can I protect the private data on my Iphone?
Take the following steps as there is no one sure shot way to get the protection that will actually protect you (at-least to some extent):


Enable data Encryption on your device, even if someone is able to gain access to your data on the phone memory, there will be a layer of protection.
Apple/Google provide web management device utilities which gives ability for remote locking, remotely wiping data, changing passwords etc, check out those settings. Some third-party app provide for this as well.
don't install apps from outside the app store.
setup secure passwords and locks (use numbers lower-case, upper-case, symbols etc, you may even use special characters that are not directly available on the keyboard)
change your passwords often, maybe every 6 months atleast.
Enable 2-step authentication. PROBABLY ONE OF THE BEST THING YOU COULD DO RIGHT NOW. Most leading service providers, DropBox, Gmail (and other google services), Evernote provide this
option. then you need two bits of information to access your
account:


one, the information you know, your password.
second, the information you get (through an SMS/Email).

Provide an option for recovery, such as secondary mail/ contact number in case you forget your password.
keep a track of security holes such as the recent Heartbleed bug.
when some one uses your phone to make a call/text, be close and it might be a good idea to take his/her picture (you don't know whome he/she might be calling to, maybe some terrorist?)
you may even want to subscribe for sms updates for your
 facebook, twitter and google+  etc notifications for any suspicious
 activities in real time.