Hey there, friend. Hate to tell you, but you're asking the wrong question. You're asking "how can I improve sales?"
What you should be asking is "how can I add value to my customers?"
More and more, people are turning to the internet to purchase phones and service plans. They're not going to set foot into a store where they think they're going to get "sold" on something and pay too much. It's just too easy to shop around and get opinions online.
What's your unique selling proposition? What do you have that no one else has? Are you friendly, involved in charity, support local business, have free wifi and comfy couches, support local artists, etc. Online stores don't do that.
Also, from now on think of yourself as an expert adviser. Give yourself some title like "the world's foremost expert in saving money on cellphone plans." Give people advice for free and send them somewhere else if it will help them better. They'll tell their friends and keep coming back for more expert advice.
Offer free stuff. Everyone loves free stuff. What about "Cellphone broken? Come in for a free repair assessment!" Write articles, tips, advice and put them in a newsletter. Pitch your free gifts in your store "Are you interested in receiving our free eReport, top 10 horrible things everyone does that kills their cellphone? Just sign up for our newsletter and we'll send it to you instantly."
Oh, and act like a normal person...not all sales-y. If you're a little scruffy, struggle to find some words, or admit when you don't know something it's actually better. And ask people about themselves - they love to talk about themselves. I like the question "what do you do for fun?"