If you are just dabbling in mountaineering and already have a rock-climbing harness, by all means, go ahead and use it for your trip. However, traditional rock harnesses, especially those designed for sport climbing, have drawbacks for mountaineering use. Ideally you'll want a harness with adjustable leg loops, so that they will fit over a variety of clothing.

Leg loops with a fixed diameter might fit well over shorts or tights, but may restrict movement when worn over a pair of long johns and waterproof/breathable pants. Ideally, the leg loops will come completely undone, so you can strap them around your legs without removing your boots. This comes in very handy when you are standing on a small ledge, wearing crampons or big boots, or if you need to answer inopportune calls of nature.

Try harnesses on with your pack, and make sure the waist belts don't conflict or rub. You don't want to feel like you're wearing a girdle at 10,000 feet, so opt for a waist belt that is comfortable and nonrestrictive. Some pundits claim that you don't need padding on a mountaineering harness, since you inevitably will be wearing thick layers of clothes. This is true, until the worst happens, and you fall in a crevasse or off a ledge. Then a nominal amount of padding could make the difference between relative comfort and agony.