Install a good antivirus, this defends your phone in an attempted attack.
Staying logged in to apps makes it easier and faster to use them, but also gives the wrong person instant access to them.
Locking your phone is a good step but adding security to certain apps like your mobile banking app or email apps can really help too.
Use strong lock screen security such as a pin or a password that nobody else knows.
If you phone ever gets in the wrong hands it is down to your password to stop that person from getting in to apps and accounts, using different passwords for each app and account makes it hard to remember the right password, but using a password manager can help you create and ultimately remember your password.
Avoid public wifi networks at all cost, they're a good and mostly free internet pass but they are also used by people for the wrong things and make it easy for hackers to access the data on your phone.