some info here
http://www.afxhelmets.com/resources/fitting.php
also, go to smf.org, snell's website.
you want to test fit for more than just "roll-off" (forward/back rotation around your head).
basically, with helmet strapped in place (room for 2 fingers between strap & throat)- HOLDING YOUR HEAD STEADY AND TRYING TO RESIST MOVEMENT, try moving helmet left to right to left. skin should move with helmet, helmet should not move much at all. now try rolling helmet backwards off head, then forwards. again, helmet should not move much (inch or 2, at most).
you should feel nothing more than light presure against forehead entire time- this area WILL NOT stretch (if salesman says it will, either he's blatantly lying, or doesn't know what he's talking about. feel free to put the helmet down and walk out of the store, or ask to speak to someone who understands helmet fitment).
wear a helmet that passes these basic tests for 15-20 minutes (yes, that long!) before removing. only (possible) acceptable pressure points should be the cheek areas- these can often be adjusted with different cheek pads, and are the only pad areas that will compress over time. i've had helmets take over 600 miles to fully break-in in this area. if forehead is sore/red, helmet is too small.
try a number of different helmet brands and models before deciding on the one you want. each company uses different shaped shells, and that can also vary across models wthin the same brand, as well.
3 nodular helmets have been/are being tested by Snell: AGV, Shoei & Nolan, iirc. if you go modular, be certain that all clasps that lock jaw to main shell are metal, not plasic/nylon.