Most important of all in buying a harness is to find one that fits you well, and is comfortable.
Try on a harness wearing the clothes you would use, not in shorts in the summer when you plan to use it ice-climbing while wearing layers. While still in the shop, you should climb, squat, walk and hang in it. It’s critical to know if you can reach the gear loops well: Rack some gear on the loops and then remove it. If you are looking for a winter harness, put it on and rack on it while wearing gloves.
Leg loops that are too tight can restrict a high step, stem or heel hook, particularly in gymnastic sport climbing. Have enough room to slip two fingers down the side between your leg and the leg loop, but it is better to err on the snug than loose side. With adjustable leg loops, you can get the perfect fit.
The snugger it is, the more comfortable a waist belt will be when you fall. It should fit well against your waist (not hips), though be careful that it will not chafe, which can be painful, especially in hot temps. Tipping upside-down in a fall in a loose, gapping waist doesn’t bear thinking of. You should only have one to three inches of space between your midsection and tie-in, and should not be able to pull it down onto your hips.
Always read the manufacturer’s instructions on how to fasten your harness properly. Buckle systems vary from thread-through types that must be doubled back to auto-locking. For a thread-through, no one can overstate the importance of doubling back, every single time, without fail, an act vulnerable to human error. Always check your own tie-in and that of your partner before either leaves the ground. Be sure that at least two to three inches of webbing poke out of the waist-belt buckle after you double it back.
Tie your knot carefully and always check it, and check your partners’ knots.