I think the most important prevention is to spend a good amount of time with your chickens on a fairly regular basis, not just hangin' out but *looking* at them, and inspecting the harder-to-see regions of at least some of them (tummies, vents, skin below feathers, feathers on upper back between shoulders, etc).
And then of course get onto any issue right away, not 'when I get around to it'.
I've dusted DE onto the coop floor before putting down bedding, but I don't actually know whether this has done anything for me or not. (I've also used it on chickens with mites, where it most certainly did work, but of course that's not preventative)
In terms of whether to worm preventively, I think you will find two distinct camps here, depending on things like whether birds free range, whether the person depends on being able to eat/hatch the eggs, and how much the owner worries about chemical stuff in their eggs.
Pat