Here's a setup I've been experimenting with for about a month now. The aim is to photograph the palps of live male spiders for identification, without harming them. All materials used thusfar are very inexpensive and easy to come by. Individuals are placed on their backs between several layers of soft tissue paper and the plastic from a CD case. In this way, the spider is safely immobilized, and we have a clear view of the palps. Beneath the tissue paper is a sheet of plexiglass supported by a couple of boxes. The reason is to be able to put a flash with an optical trigger underneath and get a little back lighting. I do everything on the floor because usually the first thing the spider does when you lift the plastic off is to right himself and dash off. It's good to have some space to catch him in!
Getting the spiders into position is sometimes tricky. I've had success putting a spider in a jar, covering the jar with the CD case, and then flipping everything upside down to get the spider onto the plastic. When he looks settled with his legs relatively spread out, I remove the jar and quickly cover him with the tissue paper. Then I place everything on the plexiglass surface, with the CD case on top so the spider's belly is facing up. One problem that I've had happen with long-legged individuals is that the spider may squirm for a second or two initially and get a few of his legs at weird angles. If it's any worse than in this image, I stop and set things up again. Maybe it's okay for the spiders since they've all been fine afterwards, but better safe than sorry.
Usually the spider's palps are oriented well for a retrolateral shot right off the bat. Getting a ventral shot is harder though, and the palps may need to be manipulated a bit. A blade of dead grass (or thin wire) slid between the tissue and CD case works wonders. It's easy to gently nudge the palps to where you want them, and it's rare the spider will resist in my experience.
With some practise, the whole process (including the shooting) can be done in a minute or two.
Here are a few examples taken using this method:
Tegenaria gigantea (same individual in this picture):
In the ventral, you can see a blade of dead grass at the top of the frame. This guy kept pulling his palp in when I tried to nudge it into position. I used the grass to hold it out.
The following were done without the back lighting:
Meriola californica (ventral only):
The smallest I've tried to do so far. This one is stretching the limits of magnification I can get with my reversed lens.