If you really want to make your helmet fit well you'll need only a few things. Foam, whether it be an extrathick liner or added in foam, glue, a Dremmel, and some time.
Add foam (or start with an excessively thick foam liner) to the inside of your helmet where you feel that you are going to need the extra padding to make contact with your head. It's OK to use too much. Then, using the dremmel or a sure form, SLOWLY begin to shave bits of the foam you have added away until you find that perfect fit. Try to make it so that the helmet liner (foam) is in contact with your head all the way around and the helmet stays on even when you don't have it clipped around your chin.
It's a little more time consuming than I'm making it sound here, but the results will make you happy.
Golden / Colorado Springs