There are many different ways to wear your cap. Each one should be tried, attempted and then made into your own. These are not the be all end all of cap wearing (we haven't even gotten to the Eros Poli Cut). Swear by the cap, or wear nothing at all, either way you will probably be fine.
Here are a few options as illustrated by Julie with our PDX Casquette (cap) and a pair of Mark Cavendish bassboat sparkling British Racing Green Oakley Radarlock's. (Oh that is funny, it comes with a cap).
The Tuck: Helmet Off or On
Some wear their caps pulled down over their ears. Our friend Joe Staples does this and he claims that it helps with aero drag. That if he can tuck those big flappers in, then he can somehow glide unnoticed through a headwind while other toil away. He also claims that this is relatively effective at keeping his ears warm on cool days.
Lufty, or Lofty: Helmet Off
Depending on where you are from you can claim that this version has a lot of "luft" or "loft." I have come to think that they mean the same thing. It is simply to say that the cap is perched atop ones head. Precariously so. The trick with this one is to get it to a place where it sits lightly on the head and still looks extremely tall. Can be primped post ride to regain its height.
Backwards Brim Up: Helmet On (not pictured)
Peter Rubijono is the first person in my life to show this to me. He always did it during cyclocross races to that he had (and I'm guessing here) the brim out of his line of sight, but still the comfort and security of headwear. This is the way that I like to go with the cap under the helmet. Somehow it just feels right. But, then again, sometimes I go with...
Forward Facing: Helmet On
Here in the Pacific Northwest it rains a lot. When that rain is really coming down hard, thee barrier that you're creating with a front facing cap is an important one. This is what keeps the rain from getting in your eyes. Or down the back of your glasses so that you cannot see. The brim, when not preventing you from keeping an even tan, is your friend.
So, there you have it. This is not all of them, I think I mentioned that before, but just a select few. Do, what you like with your cap — crap in it like Greg LeMond, tuck it into the back pocket of your jeans — or just wear it on your head like the rest of us.