Think about what you really need or want. If your current phone has a numeric keypad but you'd rather type with two thumbs, then maybe a full keypad is for you. Or, if you prefer to surf the Web on the go, consider upgrading to more data offerings. If you want to have a lot of multimedia content, like music and movies, then maybe it's time to get a larger screen and storage space.
Try to be honest with yourself about what you really require for your lifestyle. Otherwise, you may find yourself overspending on a phone just to impress friends or to buy into a fad.