If you want a preschool child to wait, then be concrete and specific about what has to happen first. Keep your part of the bargain. If you're like I was sometimes, I'd get involved in other activities and forget about what I promised my kids until they reminded me what I said I'd do. They have to know there actually will be a later time when you keep your word. Involve your child in the process.
Children are more patient when they're busy and have something to do.
I saw a mother at the library recently who was involved in doing some research who had not brought enough toys or snacks for her child. She kept yelling at the child to wait and to be patient. That's not only unrealistic, but unfair. However, one way to teach children to learn to deal with waiting is to teach them to think for themselves. Specifically, you want to involve them in the process of thinking of how they can cope when they can't get what they want exactly when they want it.