Try these three tips from the article Teaching Patience from the Scholastic website:

• Use reflective listening. Young children don’t have the words to express what they’re feeling, but you can help verbalize those emotions. In the checkout line, you might say, “I know it’s hard to wait. This is taking a long time, but you’re doing a great job waiting.” Says Leiderman, “If you acknowledge your child’s struggle, he’ll naturally try harder.”
• Keep expectations reasonable. Asking your preschooler to wait an hour for food is just too long. At a restaurant, ask your server to bring bread or crackers as soon as you sit down, and have a book or quiet game handy to keep your child occupied.
• Use a timer to help your child visualize the wait. If he is begging for a story, but you need time to finish what you’re doing, set an egg timer for 5 minutes and tell him that when the bell rings, you’ll read the book, suggests Lerner.