What will you use it for? Do you need a phone for work, for play, or mixed use? Many phones and brands cater to certain users, so know what they are for, who they are for, and make the purchase around that.
Consider the model. Did the latest iPhone just come out, or has it been a few months? Depending on the timing, a new model might be around the corner. Do your research to know when the latest models are coming out, so you can time the purchase around those dates.
With so many options out there it can be a bit overwhelming when you are looking for a new mobile phone. When the time comes to make the choice, or you are shopping for a new phone, consider some of these things, so you do make the right decision when you finally settle on a brand and model.