In the phone memory there are many personal details such as photos, numbers and addresses of friends. So it is very important to minimize the risk of such data that may end up in wrong hands!
Just as you care about the safety of your computer by installing an antivirus or firewall and you should have very good practice to use while browsing the Internet, now that smartphones offer features similar to a computer, you must also worry to maintain the security as you use them.
You should control access to your device using a PIN (Personal Identification Number). You can also set the phone to lock automatically when you spend some time (e.g., 2 minutes) and require a password (other than the PIN) to become active. You can use the unlocking service to overcome the barrier of the service provider. Some devices use a pattern that you must dial to unlock the terminal and access the main menu.
Thus, if you forget to lock it manually, you can avoid calls or internet connections involuntary also, if you leave it somewhere carelessly, prevent anyone can access all the information you store on it.
Do not open emails from senders you do not know, or download files you do not know its origin. It could, in both cases, install a virus on the device with malicious intent.
Keep updated the software on your phone. Remember that to do so, you should always use the official website of the brand of your phone. This measure should also apply to downloading and installing applications on smartphone. The operating systems currently used by smartphones, Android and iOS, for example have their own App Store from where you can download the available applications.
Encrypt your sensitive data and will not be readable by anyone attempting to access your device.