When shooting in a slot canyon for the first time it can be an exciting time. All of the excitement and countless compositions create an overwhelming feeling from the moment you step through the narrow threshold. It can really help to hone in on some more simple concepts in order to avoid becoming the photographer who holds down the shutter button hoping to capture something nice.
Texture
Keep in mind the patterns and lines of the canyon you are in. Make sense of what is going on and allow it to compose your camera. The natural lines created by water as it travels through make a good example of what you can do using leading lines down the canyon to an interesting bend or shape. You can also use the character of the slot to frame a subject deeper inside. Keeping it simple here is important as textures can be busy and challenging at times.
Movement
This is not the most obvious of the things to focus on, but it can be very dynamic the used in conjunction with the other two tips. You can use sand or water to create some excitement in an otherwise static scene. It can be falling in a soft blurred way during a long exposure, or through shortening the exposures slightly you can capture shapes and brilliant ambiance of backlit sand in the air!
Light
Saving the best for last this is my favorite bart about shooting in slot canyons. Light creates the most dynamic of images in any location, but it is something truly special in slot canyons as you are looking for the exact opposite from normal. Clear skies are ideal for creating the best light in canyons and here is why.
Reflected
When reflected light happens in slot canyons it happens in one of two ways or both at the same time. You can get the soft glow off of the walls of the canyon diffusing harsh mid-day sunlight, and there is the reflection of a blue sky in the shadows especially apparent deep inside where the spectrum of visible light starts to separate out like it does in a spectroscope.
Backlighting
Shooting into a wall of the canyon lit properly by the sun while a subject is in shadow can create an incredible light condition. The warm light from the red canyon wall in the American Southwest while photographing a subject in cool shade is hard to beat! Also, It can finally be captured in the full dynamic range of most modern cameras.
Spotlight
Yet another special phenomena popular to shoot in slot canyons is the narrow beams of light that occur. On their own they can create an awesome subject in a unique locations. This is made even more special as you can toss sand in the air to create incredible imagery while dragging the shutter. The backlit sand in the spotlight will glow as it flies through the frame. This type of landscape photography is really magical and it has resulted in several of my favorite images of all time.