Your harness connects you to the rope. It should fit well enough that you forget you're wearing it and check well enough to trust it.
Waist belt
Should sit above your hip bones, not on them. If it rides up on your ribs, it's too small. If it slides below your hips, too big.
Doubled back is non-negotiable. Webbing through buckle twice, always.
Two to three inches of extra webbing past the buckle when doubled back. Not eight inches (too big) and not a half inch (too small).
Leg loops
Snug but not cutting. You should be able to slide two fingers between the loop and your leg.
Adjustable or fixed? Adjustable for most people. Fixed for lighter climbers and alpine use. If you'll wear the harness over bulky pants in winter, get adjustable.
Gear loops
Four gear loops minimum for trad. Six for multi-pitch. The loops should stick out enough to hang gear easily without flopping.
Rear haul loop: for a chalk bag tether or a small tool. Not for connecting to an anchor.
Belay loop
The one piece you connect your belay device or PAS to. Should be stiff but flexible.
Check it before every climb. Look for fraying. When the sheath is abraded, retire the harness. This is not optional.
Hanging test
When you buy a harness, have someone hang you from the ceiling or a doorway for five minutes. If your back hurts after two minutes, wrong harness.
Every harness feels fine standing still. Hanging test finds the ones that fit your body.
Brands I trust
Black Diamond Momentum: great first harness. Cheap, comfortable, adjustable leg loops.
Petzl Adjama (men's) / Luna (women's): gender-specific cut, better for long hangs. More expensive. Worth it for trad.
Mammut Ophir: minimalist alpine-ish, comfortable. My multi-pitch harness.
When to replace
Every 5 to 7 years, even if it looks fine. UV and sweat break down nylon. Buckles can wear. Don't push it.
Immediately if it takes a fall that damages the webbing. A single impact can compromise a harness.
