When trying on a motorcycle helmet wear it at least 5 minutes and then take it off. Concentrate on places which start to bother you. If possible look in a mirror after to see if you have any red marks on your face.
Try to avoid placing your helmet too high off the ground. So often have I seen a helmet go crashing to the ground because they were left hanging on the mirror, placed on the saddle or on top of a wall. A simple fall can bust your expensive motorcycle helmet and that's just a shame.
Always make sure you understand with what helmet safety standard you are riding. You should be aware that if it doesn't meet the norm your insurance company might give you a hard time. Just so you know...
Check if your selected helmet has anti-scratch and a good anti-fog treatment. Also look to see that the visor doesn't touch against the helmet outer shell. This will result in scratches and unclear area on your visor.
A lighter helmet will of course always be more comfortable than a heavier version.
Give your chinstrap some consideration when checking your motorcycle helmet (make sure it's not too long and can be tucked away, so it doesn't keep hitting you on your chin). The strap is often made of nylon (like a seatbelt).
Ventilation systems are a major cause of noisy helmets. The more aero-dynamic the motorcycle helmet (and the vents) the less noise you will have.
The comfort liner adjusts a bit to the form of your head. Generally motorcycle helmets get more comfortable. But always make sure you do start with a very good fit.
Always inquire about the helmet safety standard. Especially cheaper motorcycle helmets try to avoid some safety testing standards.