This image has kicked around the net for a while - IDK what the original source was: Otherwise, I second the guidance and link provided by @fady.
bicycles.stackexchange.com/users/213/Gary.Ray
Snug!
It wont work if the helmet is not securely strapped to your head.
Here is a good article/post about wearing helmets.
You want the helmet to be comfortably touching the head all the way around, level and stable enough to resist even violent shakes or hard blows and stay in place.
It should be as low on the head as possible to maximize side coverage, and held level on the head with the strap comfortably snug.
http://www.bhsi.org/fit.htm
bicycles.stackexchange.com/users/499/fady
My summary of helmets.org 's fitting tips: Size the shell to your head by using/removing the (comfort) pads.
Different helmets are made in different sizes.
Place your helmet level on your head.
Typically the front rim of your helmet sits two fingers' width above your eyebrows.
Raise the Y connection of the side straps up to just below your ears
Tighten the chin strap until you know the helmet can not move or rotate without your head coming along for the ride
(but make sure you can still speak, drink from your water bottle, and so on)
bicycles.stackexchange.com/users/698/josh0
Coincidentally I brought my kids to a local bike safety event this weekend.
The rules they prescribed were: (1) If you look up you should be able to see the front of your helmet, (2)
The straps should form a "V" around your ears, and (3)
When you open your mouth you should feel the helmet push down on the top of your head.
bicycles.stackexchange.com/users/4825/Joe Ganley
First, buy a helmet of a size that fits your head (you should not be able to put a finger between your head and helmet).
Then adjust the straps binding just below the ears and then adjust the length of the strap under your chin so that you are able to move your head comfortably without tightening. is important that your front is not see, even when trying to move the helmet forward and back (Gary's picture).
-- google translated
bicycles.stackexchange.com/users/651/miguelo
I see enough folks wearing helmets in such a manner that they might as well not be.  What is the proper way to fit & wear a bicycle helmet?
How should a bicycle helmet fit?
bicycles.stackexchange.com
<safety><helmets>
bicycles.stackexchange.com/questions/1812
bicycles.stackexchange.com/users/17/Drew Stephens
You want the helmet to be comfortably touching the head all the way around, level and stable enough to resist even violent shakes or hard blows and stay in place. It should be as low on the head as possible to maximize side coverage, and held level on the head with the strap comfortably snug. Typically the front rim of your helmet sits two fingers' width above your eyebrows.  The rules prescribed by helmet.org are: (1) If you look up you should be able to see the front of your helmet, (2) The straps should form a "V" around your ears, and (3) When you open your mouth you should feel the helmet push down on the top of your head. Helmets vary in size, so you should buy a helmet that fits your head without any leeway. The helmet should be a stable but comfortable fit, set low to facilitate coverage, and strapped securely according to the instructions, so that it does not move independently of the head. 
Helmets vary in size, so you should buy a helmet that fits your head without any leeway.  The helmet should be a stable but comfortable fit, set low to facilitate coverage, and strapped securely according to the instructions, so that it does not move independently of the head.