I have always wanted to do this. Now, when I attempt this at one of your parties you can rest assured knowing I'm a trained professional!

From WikiHow:

How to Pull a Tablecloth from Under a Place Setting

This article will teach you how to pull a tablecloth out from under dishes, just like this.

Anybody can yank a tablecloth out from under a table setting. The only hard part is cleaning up all the broken dishes on the floor. Do you want to amaze your friends, entice members of the opposite sex, and keep dishes intact? Follow these steps to become the star of your next dinner party.

Steps

1. 

   

   When laying out the tablecloth, the edge opposite you should be flush with the edge of the table, while the edge you will pull should hang well over the table.
   Set up the trick. If you're just practicing, you can set up everything just right, but if you're ready to perform, you'll need to make sure you try this trick in the right situation.
   
   
   • Make sure the tablecloth is flat on the table and free of wrinkles.
   • The tablecloth should be flush with the end opposite that from which you will be pulling (the cloth should not overhang the table along that edge).
   • Let the tablecloth hang well over the edge from which you will be pulling.
2. Arrange dishes on the table. Begin with just one plate or bowl right in the middle of the table. Once you master this, add more dishes and arrange some of them closer to the table's edges until you are prepared to do the trick with a real table setting.
3. Put something in the dishes. If the dishes are too light, the trick will be hard to perform. This is especially a problem with plastic dishes. Usually, however, dishes have something in or on them at the table, so this isn't a problem. If you're practicing, put some fruit in the dishes, as this minimizes any spillage problem.
4. Grasp the tablecloth with both hands. Position yourself so that you are at about the midpoint of the edge of the tablecloth you'll be pulling.
5. 

   

   Grasp the tablecloth with both hands and bunch it up to the edge of the table.
   Bunch the tablecloth up to the table's edge. If you've laid the tablecloth correctly, you'll have plenty of cloth draped over the edge of the table from which you'll be pulling. Use your hands to bunch this excess cloth up until you reach the edge of the table.
6. 

   

   The tablecloth being pulled out from under one dish.
   Yank the tablecloth downwards and step back from the table. Very quickly pull the tablecloth straight toward the floor. As you are doing so, smoothly step back from the table, but do not pull the tablecloth outward.
7. Finish with a flourish to make your performance look even more impressive.