You get about 160 characters to utilize in your messages. Make your messages clear and to the point. The shorter the message, the more likely it is that the customer will read the whole thing.
You need to tell customers what you want them to do at the end of your message. You have a number of different options. Some popular calls to action include "show this message", "click here for more info", "reply to enter", and "buy now". Use hyperlinks when applicable to get people to the right place.
People love thinking they have the opportunity to win something. You can throw a text to win contest that will encourage customers to sign up for your messages. The people will remain on your messaging list after the contest. You can let them know about everything else that your store is offering.
Pizza Hut- "OMG Becky. Look at her Pizza Hut text message deals. Text pizza to 69488 to get unreal deals sent right to your phone."
Applegate Kitchens- "Buy one hot dog and get one FREE when you join our mobile text club. Text APPLEGATE to 585858"
You can even put the messages on receipts. Don't forget to give people a reason to sign up.
Lunds and Byerlys- "Thank you! Join Our Text Club! Get exclusive deals by texting FRESH to 95173"