Aero helmets have become very popular for triathlon and time trialling. This is a very beneficial and cost-effective way to buy a little speed for your bike split. I am not going to criticize who wears these helmets or if they look funny, but it is important to wear the helmet correctly if you want to gain maximum advantage.
The problem is that if you do not have the helmet positioned right--or if you hold your head in the wrong position--the helmet may be no more beneficial than a regular helmet. In fact it may even be less aero. So the first tip is that you must keep the helmet horizontal. When you are really pedaling hard sometimes it feels good to hang that head down (so that the "fin" sticks straight up.) Another issue is when you are looking to see what gear you are in or reaching for a water bottle. You need to keep the helmet horizontal pretty much the whole time you are riding.
Now for a more nuanced look at the head/helmet position. Here Cat is in for a bike fitting, so she brought her helmet. Notice the amount of space between the lower/rear edge of the helmet and her neck/back area. Also notice how the helmet sits distinctly
above the level of her shoulders. The key is to close up those spaces and get the head in-line with the torso.
Too much space between helmet and back. Head is too high relative to torso.
When you first put your aero helmet on during a race, I suggest pushing it far back on your head (so the fin is pointing downward. If you achieve the correct head/helmet position the brim of the helmet with be quite high on your forehead. Once you start pedaling, settle into aero position...you should feel the tail of the helmet resting against your back. Here Cat has solved that problem, but her head is still too high relative to her torso.
Space is closed off between helmet and shoulders/back, but head is still too high.
The key to the head position is called "turtling." It involves dropping your head and chin lower so it is in-line with the torso. This gets the head/helmet out of the wind and into the same streamline as the torso. One thing that you will notice is that your face is more oriented towards than the ground than forward-looking. Your range of vision should only be 20-30 yards in front of you and that is with your eyeballs looking along the very top of your range of vision. For this reason I recommend a aero helmet with a visor because if you are wearing sun glasses you are either looking at the top of the frame, or over the top of the glasses.
In this picture Cat has her head signficantly lower, but she is still tipping her head so her face is oriented forward. I would like to see her face more oriented towards the ground...this would get the head even lower.
Helmet-back space is closed off; head is signficantly lower, face could be oriented more towards the ground to get the head even a bit lower and in-line with the spine.
Hopefully these pictures and this description have got you thinking about your aero position. I am a huge fan of aero helmets. The vast majority of the fastest riders wear them, and they are a great value for the amount of aero benefit they provide. However I think a lot of people just slap them on their head without thinking about how the position ought to be refined to get the most benefit.
Happy and safe riding out there!