Helmets are not hats! They must be level on your head and strapped on securely to be protective in a crash. You want the helmet to be level on the head, with the fitting pads inside touching all the way around and the strap comfortably snug.
First, adjust the fit pads or ring
Most helmets come with extra foam fitting pads to customize the fit. You can usually remove the top pad or use a thin one there to lower the helmet on the head, bringing its protection down further on the sides. Use thicker pads on the side if your head is narrow and there is a space, or add thicker pads in the back for rounder heads. Move pads around to touch your head evenly all the way around. If you have a "one size fits all" model with a fitting ring instead, just adjust the fit by tightening the ring if needed.
Then, Adjust the Straps
Put the helmet on, level on your head. Adjust the rear (nape) straps, then the front straps, to locate the Y fitting where the straps come together just under your ear. You may have to slide the straps across the top of the helmet to get them even on both sides. Then adjust the chin strap so it is comfortably snug. Now adjust the rear stabilizer if the helmet has one.
Are you done?
Shake your head around violently. Then put your palm under the front edge and push up and back. Can you move the helmet more than an inch or so from level, exposing your bare forehead? Then you need to tighten the strap in front of your ear. Now reach back and pull up on the back edge. Can you move the helmet more than an inch? If so, tighten the nape strap. When you are done, your helmet should be level, feel solid on your head and be comfortable. It should not bump on your glasses (if it does, tighten the nape strap). If it still does not fit that way, keep working with the straps and pads, or try another helmet.
The above information was reprinted with the permission of the Bicycle Helmet Safety Institute. For more information go to www.helmets.org.