Shop at a mobile phone company store, not a kiosk or booth at a mall or department store.
Talk to a dedicated salesperson, someone who knows the product well and can help choose a product that is right for you. If you have access to a business sales person, oftentimes they can offer you better service and plans customized for you.
Don't be confused by features in a plan. Buy what is right for you. No matter how cool the salesperson makes the various features sound, buy only what features you will use.
Take your time and ask questions.
Don't be afraid to take information away, read it at your leisure, and return when you're ready.