The backup which iTunes stores for your iPhone every time you sync it includes your contacts. So, even if you get a replacement phone you should still be able to restore from your backup of the current device.
There are also several app store apps which will backup your contacts to various different locations. One free one I have used is "iDrive Contacts". You just install the app, create a free account, and it lets you backup to their servers. You can then install the app on another device, log in with the same details, and restore. Very useful for sharing contacts between multiple phones!
But, if you don't want that hassle, iTunes standard backup should be fine for your needs. Just make sure you sync once in a while to update your backup file!
Hope that helps.