Now that you have their contact information, it's time to make contact! Although you don't want to sound plastic or rehearsed, be prepared.
If you get them on the phone, just be genuine and upbeat like when you met. You may want to remind him/her of who you are. "Hi John, this is Nancy. We met last weekend at the concert". Now, continue as you usually would with any conversation, "How are you?". A little small talk is fine, but you'll want to keep the call brief and to the point. Your goal is not to get to know them over the phone, it is just to set up a date.
Be prepared in case they don't answer. If you get a voice mail, be sure to leave a message! You don't want to be a pest by calling more than once. Leave a lighthearted message and be sure to leave your phone number. If your number wasn't stored in his/her phone and you didn't leave it, there's no way to call you back and you'll be left wondering what happened. Here's an example of an appropriate voice mail to leave:
"Hi John, this is Nancy. We met at last weekend at the concert. I can't believe you've seen that band as many times as I have! Anyway, I am hoping you'd be available for lunch or dinner this Tuesday or Wednesday. Give me a call at -------. Talk to you soon!" Mention in the message that you would like to get together. This way when they return your call you know they want to get together.