Lock in your idea and your shadow patterns from the beginning and never chase the light. You only have three hours at the most on ckear days depending on the angle of the sun in relation to the painting. maybe five hours on a heavily overcast day. Start with small size canvases and a big brush. pay attention to saturation and hue shifts as well as value. I have a number of demos on my blog in step by step process you might find them useful. Scroll down on th eblog to see the plein air ones (the first two are a digital painting and mural )
I've attempted a couple of plein airs to little success lately, and I'm wondering if using a viewfinder would help me to focus my efforts. Does anyone do this?
A viewfinder can certainly help..but I just use my hands and make a frame with my thumbs and pointers at right angles. Turn your left hand backward to make it easier.
The important thing to do is step back from your easel, "frame up" your support/canvas pretty exact, then move your frame out to the view. "Zoom" by moving hands toward/away obviously. The first important thing to note is the horizon...where it sits in the frame. Then the big shapes/masses and also where elements leave the frame.
You'll get the hang of it - easier done than said.
What would Caravaggio do?