Apple's OS X Mavericks is now available for free from the Mac App Store, and if your system meets its minimum requirements then you might be considering installing it. However, you may be concerned about whether you need to prepare your Mac for the upgrade.
Check the specifications
First, the only true requirement for upgrading is to ensure your system meets the minimum system requirements for running Mavericks. If your Mac can run OS X Mountain Lion then it should run Mavericks, and in its announcement today, Apple reiterated this includes the mid-2007 iMac, MacBook Airs from late-2008, and the Mac Mini from early 2009. Also keep in mind the App Store will check for system compatibility, so if you can download it to your system without error, then it should install just fine.
Clear up existing problems
Next, ensure your system is not experiencing any major problems, which include regular freezes or crashes of applications or the entire system, or the inability to log in to a user account, enable sharing services, or establish hardware connections to peripheral devices or networks. Sometimes these can indicate hardware problems, but otherwise may suggest software configuration errors that may migrate with the system during the upgrade. Granted the refresh of OS software may in fact clear some of these problems up, but it may be more assuring to install the new OS over a known working system configuration.
Run basic maintenance routines
Another step to consider is to run a few maintenance routines on your hard drive before installing. At the very basics, reboot the system into Safe Mode and then use Disk Utility to check the hard drive for errors. If problems are found, then reboot to the OS X Recovery HD partition (hold Command-R after hearing the boot chimes at startup), and run Disk Utility's repair routines there. While you can run a full general maintenance routine using third-party tools to clear system caches and temporary files, along with resetting the PRAM and SMC on the system, these steps are often unnecessary for preparing a system for an upgrade.
Update third-party software
Another commonly overlooked preparation step is to ensure that your third-party software is fully updated. If you use specific tools and utilities often, be sure to check with the developer for compatibility with Mavericks before you upgrade your operating system. All too often, people upgrade their systems only to find that their programs now do not load, or regularly crash, only because they have not been updated. This is true for small independently developed tools, and also for major software titles, so be sure to launch these programs and run software updaters, or visit their developer sites and ensure the updates that are available are Mavericks-compatible.
Finally, the most important thing you can do before upgrading is to back up your system using a fully restorable backup solution like Apple's Time Machine or a third-party system-cloning tool. These options will ensure you can recovery your system as-is if something goes awry during the upgrade. In addition, having a backup allows you to optionally perform a clean install of OS X Mavericks and then migrate your data and programs from the backup, instead of only installing Mavericks over the current installation.
While you can attach any external hard drive large enough to accommodate your data, and back your system up to it, consider using at least two backups. For one you can use Time Machine, and then a cloning tool for the second, or even make two clones or two Time Machine backups. Either way, duplicating the backups will ensure redundancy in case one backup drive fails at the crucial time of need.
With your backups performed, maintenance completed, and system requirements checked, next be sure to unplug your backups from your system to prevent them from potentially being tampered with during the OS installation, and then download and install Mavericks from the Mac App Store.