For the most part, what one wears while cycling has to fit appropriately. This of course, does not apply to those irrationally comfortable with the possibilities of coming to a thespian tumble as the result of loose hanging garments entangling in the nooks and crannies of their bike.
Perhaps the most important fit of all garments is the one that serves to protect and defend the skull from forces unwanted, the Helmet. Not only does wearing a helmet that does not fit render it near useless, it also gives off an annoyingly awkward appearance. In order for a helmet to protect the skull and its chief occupant at maximum efficiency it has to be worn with proper etiquette. An etiquette not hard to attain, only requiring the exertion of effort and the ringing reminder of three mnemonic words: Low, Level and Snug.
Often referred to as a “lifehack”, the phrase takes on its epithet in the most literal sense. Applying what's suggested in the phrase might actually save your life, let alone make it easier. “Low, Level and Snug” refers to a dreadfully easy test used to verify whether or not a helmet fits on the head correctly. When adjusting a helmet on a head, it is crucial to make sure that it fits as low and square on the head as possible; Molest the helmet from front to back then side to side in order to reassure that the inner foam surface makes contact with as much head surface as possible. Next position the helmet so that it is level with the head looking forward; the helmet should be parallel to the ground. Assure that the helmet does not lean back or forward at an angle that intersects with the ground. Lastly, fasten all the straps on the helmet so that it hugs your head. The straps should be snug, tightly but surely allowing the entrance of a finger in between. Make sure that the “Y” shaped intersections formed by the straps rest just below the ears.
Those are the fundamentals of achieving proper helmet etiquette. The order of the steps are nonspecific, but all three are essential in achieving said etiquette. Once you’ve purchased a helmet that best aligns with your head size, take into consideration “low, level and snug” every time you grab your helmet. It is important to test for such etiquette every time you ride due to the fact that as time passes, things, especially straps, loosen and become undone!
Here's a link to a beautifully rendered brochure of proper helmet etiquette: http://www.getstreetsmarts.org/downloads/resources/SCV-3064_Helmet_Brochure_EN_mech.pdf