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 skill that has been built up through hours of practice.
The Technical Bit
The ollie1 is performed as follows: the board is placed flat on the ground. The skateboarder puts their back foot on the tail of the board2 and their front foot 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. They will then proceed to slide their front foot towards the nose of the board while simultaneously jumping upwards. If the board wasn't there it would almost be like doing a martial arts-style kick. The sliding of the front foot forwards will cause the board to level out. This is pure physics, as by pushing the nose of the board downwards, the skater is causing the board to rotate around its centre. The skater's back foot stops this rotation so that the board levels out.
Coming Back Down
To land the ollie properly, the skater should extend their legs and place their 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 they 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 — in other words, 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 it is generally agreed that the more skilled a skater becomes at home on the carpet, the lower the chances of injury when trying to do the trick at speed.