In foods class, they told us we could learn by practicing with a piece of toast. That is how i'm going to show you. I've never tried it, but, my dad said that he learned (at the Culinary Institute of America) with frozen peas.
Step 1: What you need
One piece of bread
A toaster that doesn't want it's picture taken
and food you actually want to eat (like veggies)
Step 2: Toast your bread
Step 3: The Technique
You push forward, tilt up, bring the pan up a bit, and then pull back (as you catch the toast). Just keep trying this until you get it down, and are able to do it successfully every time. Adjust your technique as you see what happens.
Step 4: Tips for real cooking
Make sure the pan is greased well enough for your food to slide easily, this will make the flip easier.
When you are just starting, a "fold" is good to try, so you can get used to it, but not ruin the food or get burnt.
If you are doing something like eggs, make sure the egg is solid before trying to flip it.
Sandwiches shouldn't be flipped too much, or they will start to break. They are a good thing to practice with though (grilled cheese)