A little photographic trick: Look for opportunities where the fly-caster has a dark background behind him, or at least a patch of dark, across which his fly line will pass when he casts. You might have to ask him to step forward out of the shadows just a little in order to get the sunlight to catch his arcing line. Then take pictures on continuous shooting , in order to get the line at the perfect spot. These opportunities will present themselves more in the early morning or late afternoon, and more so in steep river canyons, where shaded vertical walls are common.
In this first image, I had to wait for PD to fish the run properly (he was hidden in the shade), and then I asked him to step forward just a step or so to get the line into the sunlight. We were on the Riflespruit, and it was the first pool of the day.
The picture is special because my father, an artist (oils), painted the scene, and PD now has it hanging in his house.