Use Your Head!
Software is a valuable defense against smartphone scammers and thieves, but it's not foolproof — you still have to use your head. Here are some important points to keep in mind:
Use your phone's locking feature. Your phone gives you the option to require inputting a PIN or password before gaining access. Yes, it's a pain, but if you find yourself frantically patting your pockets for a lost phone, you'll be glad the lock is enabled.
Don't fall for fraud. Many of the same caveats I laid out for Facebook users here hold true when using your smartphone online. Avoid too-good-to-be-true offers. Don't trust questionable sources — if you need to contact your bank, for example, go directly to their own website address, or make a phone call, rather than clicking on a link in a message. Make sure you have a secure encrypted connection when using Wi-Fi — the browser bar will indicate whether or not the connection is secure.
Check permissions when installing an app. Every time you install an Android app, you get a pop-up window explaining what phone features that program will be able to access. Don't just click OK — give it a read. Is there any reason a game should have permission to send text messages, for example? Should a recipe app have permission to access your contact list? A little common sense will go a long way toward avoiding serious security problems.
Read the reviews. While Google doesn't test apps before making them available in the Android Market, it does let users review apps they've tried. Take a look at what others have experienced with a given program before downloading and installing it on your phone.