I use little rubber-covered wire chip clips; 2 or 3 down the back to cinch it, but you gotta take pics soon lest better fabrics will show wrinkles where you clipped it. Not my best pic (was still getting lighting worked out, and I should have steamed out some wrinkles), but an example. The shirt is longer than the coat and my mannequin is jersey-covered Styrofoam, so I do pin that up and under. EDIT: you were probably asking for pics of the actual pinning...sorry, I'm too lazy to set one up right now, but imagine the fabric pulled somewhat tight down the back and gathered with some small chip-clips. I sometimes do the same down the front when taking photos of the back, but usually not unless it's a really big size. Pinning probably takes a bit longer, but probably also looks better overall.
Also, someone asked earlier about taking pics of a suit and how to include the pants - this is how I do it. I've seen other sellers drape the pants over one of the shoulders which looks good as well, IMO.