Lichen are visually fascinating: their bodies have gorgeous textures, lines, and colors. But capturing all of that can be challenging. Take, for instance:

That lichen has gazillions of lines and textures on its body, but the diffuse shade lighting isn’t helping show them 1.
I typically seek out lichen that are in the shade, but in an area that has full sun available only a few inches away. This means I can hand-hold a small reflector 2 to bounce sunlight back onto the lichen, trying different angles as I work the camera. Here’s the exact same lichen with reflected light added to the shot:

In that image I’m reflecting light in from the left of the image, and holding the reflector a good distance away from the rock (nearer to the camera) to shorten and soften the shadows. The textures of the lichen are brought out beautifully.
If you want to add more lighting contrast, just hold the reflector closer to the rock so the light comes in at a shallow angle and the shadows get less fill:

In that shot the reflector’s now to the right of the lichen, and lower to the rock. The lichen looks, on first glance, completely different. In fact, it looks so different that I included two pictures of the same lichen in my introduction to lichen post; did you notice they were the same?
And, for your comparative pleasure, here are all three images stacked up:

Here’s another example, this time from a lichen in partial sun:

I find the strong highlights of the sun on the rock and lichen to be distracting. Using reflected light brought up the shadows to more closely match the level of the highlights, and also brought out more of the texture of the lichen:

Note that the sunlit highlight on the lichen is minimized, as the entire body has been brought up to (or past) the level of that spot (this shot has about about 1.67 stops worth of light added). But, of course, hand-held reflectors can move, and so I took shots with two other angles of light.
Left:

Top:

I settled on the last shot in the sequence (reflected light from above) as my favorite, but they all have their high points 3.
And here are all four images together:

Doing it:
Carrying out this technique in the field is easy, but unless you’ve got five extraordinarily steady arms you’ll need a tripod, and probably a cable release 4. Seek out a specimen that’s visually interesting, and if you think the lighting can work as a base, set up your tripod, settle on framing, focus, and take a few test shots. Then start playing with lighting 5, trying both sides of the reflector and a variety of angles; the lichen may surprise you with what works best. Sometimes I settle on lighting right away in the field, and other times I shoot a number of lighting options and compare them during processing.
Footnotes:
1 When you find lichen, they’ll be on some substrate (wood, rock, or soil) in one of three lighting situations: full sun, partial sun, or shade. Full sun can be harsh, with too much dynamic range for a sensor to capture. But depending on the angle and type of lichen full sun can sometimes can work out well, highlighting the textures without casting overlong shadows or blowing out highlights (see, for example, this shot). Partial sun is typically difficult, unless you can soften the shadows somehow. I often focus on lichen in the shade; in the shade all of a lichen’s body is fairly evenly illuminated, and shadows are likely to be mild. This is the perfect base for photography, but evenly diffuse lighting tends to minimize the three-dimensionality of lichen, hence the reflected sunlight.
2 I use a hand-held 14″ Wescott white/silver reflector. Hand-held reflectors give you great flexibility. If you want just a little bit of light to bring up the shadows, use the white (diffusive) side up close, or use the silver (reflective) side and hold the reflector a couple of feet away from the lichen (or aim the reflected light so it lands a just off the lichen). If you want stronger/harsher light, hold the reflector closer to the lichen and/or reflect the light so it’s more parallel to the lichen’s body. And, of course, play with angles!
3 Oddly, when I look at the left-illuminated image, sometimes the lichen looks like it’s embedded into the rock rather than growing out of it.
4 My full gear for shooting lichen right now includes a macro lens (Canon 60mm EFS), tripod with a ball head, cable release, polarizer, 14″ Westcott Illuminator, and a ruler (those scale bars don’t make themselves!). A macro slider would probably make things easier.
5 Since I’m varying the light level dramatically during this testing, I typically leave my camera on aperture-priority mode while I’m playing with lighting angles and amounts. Once I settle on lighting, I switch to manual exposure to facilitate exposure blending and repeatability. One thing I’ve noticed on my 30D is that light coming in the eyepiece can sometimes mess up the metering, so I try to block the eyepiece with either my eye or something else until I switch to manual exposure mode.