Do not divide them in the fall do it in the spring. Use your photo as a guide to tell you just where you want to plant clumps. You can even take more photos of where you want to put them. This is the easiest way to divide perennials, because you can see exactly what is there, what you want to showcase and not hide, and where you want them.

In the spring when they are just coming out of the ground dig them up--you may not have to dig all of them. Sometimes you can just shear off a peice.

Oilpainter,
Thank you for your advice; however, these plants have never died back in my garden. They are green all winter which means when I divide them, I'm going to have tall lanky pieces. Do I cut them back to the ground? This will remove most, if not all, of the leaves.

I also divide my shastas in the spring, mine also stay green all winter. I have several varities some yes do stay lanky after the divide and some get a little more full by the end of the year.

This year I plan to attempt to split some of mine this fall. I have never split my shastas in the fall but I am going to try it this year since our weather has been so weird lately. My suggestion is not dig up the entire plant just the part you want to thin out. I take my hand shovel put it approximately where I want to seperate it from the rest of the plant. Thats where I dig, so that the main part of the plant will not have to suffer through the shock along with the part I seperated.

Oilpanter is right about taking cuttings, shasta's root extremely easily from cuttings. You can do the cuttings during the growing season of the plant. Though most shastas totally benefit from a complete split every so often. Shasta's are supposedly a short lived plant. Its advised on some varities to actually split every 3 years and remove the middle and disregard, since the majority of the blooms come from the "new" shoots.

I personally think dividing in fall will be ok since I not only notice my plant to stay green all year but I also notice new growth in the winter months. I would be curious to hear from others in warmer zones to find if they have tried splitting their shastas in the fall.

If they are too lanky after you divide them trim them back. You people may be fine dividing in the fall anyway since you are in a warmer climate. I'm in zone 3 and mine always died back.
Just don't wait too late so the roots have a chance to settle in before heavy frost.