The wall ride is one of those tricks that look harder than it is. It does require full commitment to pull it off though; otherwise it'll be less wall ride and more face slide...
There are two types of wall ride. Some are quite vertical, which you can roll on and off of. For others you'll need to pop on to them and jump off for a smooth landing. They both draw on the same principles though.
Attack the Wall:
1. Angle of attack - With left foot forwards, attack the wall at a 2 o'clock angle. Too far towards 3 o'clock and you risk sliding down the wall too far towards 1 o'clock and you'll bounce off.
2. Pull up - Most wall ride options will have a bank beforehand that you can use to pop on to the wall. Lean back, pull up on the wall, and prepare for impact.
3. Carve - As soon as you're on the wall, you need to start your turn, to maintain grip on the wall. A longer carve requires more speed to stay on - otherwise you'll lose grip and slide down.
4. Off the wall - Even if it's a wall you can roll off; unweight the front wheel by pulling up off the face. This isn't about looking flash - it will allow smoother landing.
Things to consider:
1. Look where you want to go - On your approach, you spot your take off. As you're pulling up on the wall, spot your exit point and check out your landing as you carve.
2. Mind your speed - More speed equals a longer wall ride, because you increase the grip of your tires. But don't hit the wall too hard - a big impact will bounce you back off again, so start slow.
3. Best foot forwards - Typically if you ride right-foot forwards you'll prefer to wall ride left to right and will need to attack the wall at approximately a 10 o'clock angle. If you ride left-foot forwards you'll most likely prefer right to left, and a 2 o'clock approach angle.
4. Look for a Landing - Landing from a wall ride can be harsh, so try and spot a small backside you can land in to a minimize the impact.
Bike Set-up:
1. Tire Pressure - Running your tires softer might offer more grip, but wall riding is a dynamic move, and puts a lot of stress into the wheels. A firmer tire will give better support and a more predictable ride.
2. Suspension set up - Softer suspension may absorb the impact of getting on and off the wall, but it will make those actions harder. Try firming up your suspension to a quarter of available travel as sag - or less - and go from there.
3. Saddle height - You need to move around on the bike a lot, so get the saddle down and out the way. Even when you've mastered the wall ride, bigger and steeper ones are hard to do with the saddle at full height.
Practice Makes Perfect:
Even when there's nothing as extreme as a wall ride on your local ride, chances are there will be some steep banks at the side of the trail along the way. Practice the same technique, and learn what doesn't work by finding out where the grip is. You'll find quite often you can wall ride sections of bank to avoid roots and puddles.