Once you've decided on your phone budget, operating system, carrier and plan, you've got one task left: deciding on a phone. Take your time on this. You may end having the phone for two years, taking it everywhere with you and using it more than any other appliance. You need to make sure the actual phone suits your life.
Check out all the phones the carrier you're going with has in your price range and with the operating system you want. Think about the tasks your planning on using your phone for, and try them out on the demonstration phones in the store. See if the key board is comfortable, and how easy it is to switch between applications. If the phone has a trackball or touchscreen, see if it's easy to use or if you're constantly clicking on the wrong thing. Make sure the phone will fit comfortably in your hand, as well as your pocket or purse. Also make sure you like the look of the phone -- after all, it's going to be your constant accessory. Put the phone to your ear and make sure that's comfortable as well. If you know people who have a phone you're interested in, ask them about it. Also read user reviews online to see if the phone has issues like getting hot after prolonged use, or breaking easily.
Follow these tips and your next smartphone won't be just smart -- it'll be a genius.