Last year, I went my usual route for seed saving: throw the drying stems in a paper grocery bag. When mice infiltrated my shed, I tossed the coriander seed into a new garden bed instead of harvesting possibly contaminated seed for culinary uses. I got a big, beautiful bed of coriander/cilantro. This year, I did not want to make the same mistake, and I began processing the seed yesterday. Until I can make my own screen for winnowing (something like these), I rubbed the seed back and forth over an unused door screen and let the wind take away a lot of the chaff. Of course, I made sure to save some for the cilantro bed.
The flower seeds mostly get taken care of by the birds (look closely to spot the goldfinch on this sunflower). Unfortunately, rampaging squirrels have since broken all the sunflowers off.