Start on a slanted wall, something that lets you ride up progressively like on a berm, then move on to steeper walls, but still think of it as riding up a curved berm.
Try to be smooth and progressive to maintain your grip
1° Like for a bunny hop start at a good speed, the further and higher you want to roll, the faster you must go to build momentum and defeat gravity.
2° As you get onto the berm, tilt the bike to make it roll as much perpendicular to the surface as you can and start crouching progressively.
3° As you roll higher and slower, follow an imaginary parabolic curve, and crouch further against the bike to maintain pressure and grip.
4° Once you've passed the highest point in the wall ride, extend progressively from your crouching position to maintain pressure on the wall.
5° Now if it is too steep to simply ride down, spring up while pulling on the bars to take off the front wheel and eject from the wall.
6° As soon as the rear wheel loses touch, re-adjust the tilt of the bike to the vertical and realign the bike under you to face the exit and land clean.
See this move in a video
Some tips on maintaining your grip
Basically, think of the wall as a steep slope like in a sharp bend on a BMX track. You must build enough momentum towards the wall to feel like you would fall into it if it wasn't there. By combining this momentum with the tilt of the bike, it nearly feels as if gravity was not pulling you vertically anymore, but sideways. To maintain that effect and keep some grip on the slanted surface, you must crouch as progressively as you can until the highest point of the wall ride. When you reach that point (that's when the bike stops going up and starts loosing grip, you'll know it), spring up progressively in order to maintain your tyres stuck on the slope and still rolling. Don't smack the wall rigidly with your wheels, this would eliminate completely any momentum towards the wall that is your only hope for a decent grip (friction).
Why a parabolic curve?
Without securing some grip with your tyres, you won't be able to roll and control the curve path. Gravity will simply pull you down too quickly and crash you to earth before you get a chance to ride along. Basically, when you jump in the air, you naturally follow a parabolic curve and when you ride along a wall, it is like a kind of jump assisted by the wall. But you are only assisted (the grip is too weak for you to stay up there), so to make it as smooth as possible, follow the nearest path to what would be a normal non-assisted big jump: that natural parabolic curve. Depending on the texture and grip of the wall, you'll be able to cheat a bit, but turn down and push back in extension again as soon as you lose grip. You'll have grip enough to roll and control the bike as long as you'll be pushing the bike on the wall. As soon as you eject of the wall ride, pull on the bars to force the bike down under you and tilt it back to the best standing position you can (once you leave the wall, gravity gets back in full control, no cheating). If you lose grip too soon (or eject too late), you'll end up either scratching your elbows or falling flat on your side.
Slamming a vertical bunny hop...
A dead end for cool pics.
The vertical stuff
That becomes a tricky ride, at least if we are talking about a proper vertical wall. Since you can't roll up onto the vertical surface, you'll have to launch a bunny hop while riding nearly along the wall, and tilt the bike horizontally to bring your tyres onto the wall. If you slam your wheels too vertical and direct, you won't be able to ride a curve. This is dangerous stuff. Unless you know how to twist back off the wall and avoid a crash on your back, eject yourself and try to land on your feet. But if you manage a kind of slanted bunny hop, use the same progressive grip-in, grip-out approach as before. Try to absorb the impact very progressively so that you don't lose grip straight away. That is what will let you roll. Remember, no grip / no ride.
Just riding along the metal plane
José pulling big moves on his 24" street bike.
(photo by Sebastian Romero)
Not only for street
With faster 24" street bikes, some riders can use small ramps to jump up to a wall, roll a bit, and push back from it to land clean. That makes spectacular moves and photos. Once you understand the tricks that keep your tyres well stuck onto very steep slopes, you'll be able to ride up some near vertical rocks, until you reach a more horizontal spot or some edge to hook the front wheel to. It will also help you for rolling up big obstacles. Your limiting factor for most wall rides is the speed at which you can approach the obstacle, depending largely on run up distance and gear ratio.