A smartphone is a small computer in your pocket, and all of the vulnerabilities mentioned throughout other sections of this guide apply. The solutions are more or less the same as those for a computer, only with different software. In most cases your smartphone will be running a version of the Android or iOS operating system. The Security in-a-box toolkit has excellent recommendations on secure mobile and smartphone usage
One major difference between a computer and a phone is that the latter always gives away its location to the cellular network. You can read more about this in Can I be anonymous whilst using my phone. Wikipedia has a useful description of various Mobile security issues. Smartphones in general require just as much attention security-wise as your computer.