In both instances, hop and bob about like a prize fighter to make sure the helmet stays in place. Bring along your own head lamp, and make sure it fits over the helmet securely, and that the weight doesn't cause the front of the lid to slide over your eyes. Check to make sure the strapping system feels comfortable when adjusted snugly around your chin. Many helmets come in Small, Medium, and Large sizes, and a few manufactures make children's models as well. Try on a half dozen designs, and buy according to fit and comfort.
Helmets come in two basic varieties.
Under impact, the shell might crack or bend, then the harness stretches to absorb and dampen the remaining energy of the blow. A couple of years ago, lighter, foam core climbing helmets were introduced to the market, looking more like bicycling attire than the Coal Scuttle Helmets, the height of fashion on the Hindenburg Line. These newer helmets shave several ounces off their bulkier brothers, but aren't as durable. However, with a UIAA rating, either model is appropriate for general mountaineering. Note that suspension models are still the recommended choice for waterfall.