People who need instant gratification are said to have Poor Impulse Control. If your child's behavior is found to be a result poor impulse control, self control will need to be practiced. Sometimes this requires a lot of time and patience and other times you may be able to say "think about what you are doing, is this the right choice?".
Example: Johnny must grab something he sees if he likes it. Make him go back to the other side of the room and walk past the object without grabbing. If he grabs it again, send him back to try again. After many tries he will learn to make the right choice. I find this especially effective when he is on his way to do a preferred task, and he doesn't want to waste his time making bad choices. This can turn into an avoidance technique if he learns that he must try again until he does it right, in that case it should be more important to finish the necessary task.
Example: Johnny gets excited about seeing a fire truck and starts to laugh and yell. He will not stop laughing no matter how much you tell him it is time to be quiet. Show him a picture of a fire truck and practice an appropriate reaction. You can model the behavior. Look at the picture and say (overacting) "this fire truck makes me feel silly, but I know that people get annoyed when I laugh too much, so I am going to smile, laugh a little bit, then take the rest of my laughs and put them in my pocket to save for later!" Physically take your hand to your mouth and pretend to grab your laughs and then stuff them in your pocket. I have found that putting things in your pockets such as humming, laughing, and yelling are very effective because it makes the sound into a tangible thing that can be gotten rid of.
