After teaching a bunch of clinics, and finding similar issues with people and their bikes, I decided to write up some tips to help the "average Joe or Jane" improve upon their bike set ups. I have spent a lifetime tweaking on my own bikes, changing stuff, testing, watching video, studying form, etc. so I would like to think I have a keen eye for biomechanics. There is a lot of info that can also be discovered from trial and error :). My tips are guidelines as everyone is different, but there may be a few things in here that can help you with your riding and bike set up...so enjoy.
#1- Bars, levers, shifting. This is where I start with people, mostly. Bar width is such a major component for trail riding and gravity stuff. Having too narrow of bars, the wrong roll, or height, can effect your form and how you corner. Just a basic non-scientific rule I think works is doing a push up on the ground and measuring the edges of the outside of your hands. It's a Strong position and custom to you. I run 740 mm to 730 mm width, 30 mm rise to 38mm rise depending on which bike. Bigger guys run wider bars than me obviously. Too wide, and you will not really have the ability to leverage the bike though.
Also having the brakes positioned too far up or down, or the reach too far out , is funky. So you want to sit on your bike in a neutral standing up position, wrists in line, and elbows up. Start with the bars and ignore putting yor fingers on the brakes at first. Loosen the bars, and try a few different positions. Make sure they are centered too with measuring tape ().
Usually you want your bars the same angle as the fork, so look from the side. See what happens when bars are rolled too far back or too far forward. Elbows go down if the bars rolled back your lap, or too high sometimes. Too far forward and your position is off, it's harder to get behind the bike in corners etc. That's my main beef with long stems, you probably aren't as good at cornering, but like it for climbing? things have changed, bike geometry has changed. A compact cockpit is really enjoyable and better for riding down hills. Different bends of bars come in too. I use Renthal Fatbars, which are perfect:) Some bars have massive backsweep and that kinds sucks too for body position unless you are really small and your bike is too big. Stem rise and how tall you are are important considerations fro bar height. On slalom and dirtjump higher bar rise.
Then I guess stem length. 40mm to 70mm for a trail bike depending on your bike. Jeese. 70 is still gonna put more weight over your front end. I run 50 on all bikes 45mm on dh.
As for lever position, once you got the bars all sweet, you want your index finger and wrists all in the same line. Some dirtjump or bmx guys run the lever super far down, but that's really only good for flat ground riding. the steeper the terrain the higher the lever to keep things neutral and natural. Too high and your elbows will drop and your wrists will be all cocked, which isnt really the strongest or safest way to ride. Sometimes we put a spacer under the bars or drop the fork out on steeper trails too to counter the weight distribution. Steep terrain will put more weight on your front end and it's hard to keep your front end from diving or your head up to see where you are going, so that's the reason for the adjustment.
Back to lever reach. This is the main thing I see with chicks. 2 finger braking is usually because of not being able to reach the lever, or it's too close to the grip. All good brakes will have a reach adjustment. So wind in the lever enough so the very end of the lever is at the end joint of your index finger without doing anything. Having the brake too close to the grip can effect your elbow too, so slide the position of the brake to a good spot so you hold on to the edge of your bar, elbows are up, and your index finger is on that knuckle. I use calipers to set them equally and the point of contact equally, and the position away form the grips equally.
Shifter should be in a place that you don't have to move anything except your thumb to engage it. Shimano XTR is super dialed. they made shorter brake levers and longer paddles on the shifter so it's obvious which side goes where. levers closest to grips. I have to changeold shifters around for chicks sometimes to accommodate small hands, but I'd say bikes are getting better and people seem a bit more savy lately at the intermediate level.
Suspension is really important as well, but It's tricky to explain and limitless really. My other big tip is to balance out your body and have good hip mobility. Adductor/abductor balance and core.
This bikeradar link is oldschool, but references some photos to know what I am talking about.