Kids like to know what's going on just as much as you do; they like being prepared and informed. Set the ground rules before you go anywhere or do anything. Use this strategy when you can, and you'll see things go smoother with your kids.
For example: "Jonah, we're going into the toy store to buy Simon a birthday present. We're not buying you a present. You can look around. You can tell me what you like. But we're not buying you anything today. Today is for Simon. Do you understand?"
Don't be surprised when your child protests, whines, or still comes up to you with a toy, explaining that he's always wanted this special LEGO Millennium Falcon, and if you get it for him today, he never will ask for anything again.
Your response needs to direct your child back to the original conversation about Simon's birthday and explain that you both had an agreement. You'll be in big trouble whenever you give in to the whining and the major fits. Your kids won't believe you the next time, if you tell them they can't buy anything and then give in and get them that one little toy. Buckling under the pressure only leads you to more tantrums because, obviously, the tantrums work.