To the unassuming bystander the act of getting a skateboard to lift off the ground without the use of a ramp may seem like magic, but it is in fact a not too complicated matter of physics, action and reaction.
It may look like the board is attached to the skateboarder's feet by invisible straps or some type of super strong glue, but it is in fact due to the skateboarder's skill that has been built up through hours of practice.

The Technical Bit

The ollie¹ is performed thus. The board is placed flat on the ground. The skateboarder puts his back foot on the tail of the board², his front foot is placed about a quarter of the way back from the nose³. The skater then pushes down with their back foot, thus causing the nose of the board to rise. This happens because the board pivots around the back wheels, almost like a see-saw effect. He will then procede to slide his front foot towards the nose of the board while simultaneously jumping upwards. If the board weren't there it would almost like doing a type of ninja kick. The sliding of the front foot forwards will cause the board to level out⁴.

Coming Back Down

To land the ollie properly, the skater should extend his legs and place his feet over the bolts on the skateboard. This should absorb the impact of the landing and prevent the board from rising up and hitting him in the face or going in some other random direction.
The size of the ollie depends on how far back the front foot is at the start of the trick and how high the skater jumps while performing it. The skater should preferably be in contact with the board at all times, otherwise he will lose control of the board and more than likely end up in a heap on the ground.

If you follow the above instructions succesfully, well done you've just performed magic. The most preferable method of practice is commonly known as carpet skating, i.e. standing still on a carpet. This minimises the chance of falling, and if falling is unavoidable it lowers the chance of being severely hurt. Some people would say this defeats the purpose of skating, as you do not get the rush of performing the trick while moving at speed. But this researcher feels that the more skilled a skater becomes at home on the carpet the lower the chances are of maimings occuring when trying to do the trick at speed.

Enjoy yourselves, try not to get too hurt.