Tidy Up Your Data
The first step you should take to prepare your phone for use by a new owner is to tidy up the data. At the very least this will mean finding a way to copy your contacts to your SIM card or syncing them to a computer or cloud service before resetting the phone to its factory defaults.
However, you might have other data on the phone, such as photos, videos, MP3s, or even games. You might have used your handset as a portable storage device as much as you used it as a phone, and being aware of what data is stored where is vital if you want to keep copies of these media files, documents you have been working on or saved game progress.
Various tools will be available for you to take copies of this data. Often, mobile phone manufacturers make software available that can be used to “explore" a phone from a computer via a USB connection, and this is the best way to keep a copy or backup any data from your phone that you wish to keep.
Before selling your phone, also remember to remove your SIM card (for safe-keeping or to use with your new phone) and any expansion card (such as a microSD) that you might have installed.
Once you have backed up all that you wanted to keep, you will be ready to perform a factory reset.