Fiona:

I just read your question and I'll tell you what we do at my greenhouse.  We grow hundreds of sunflowers for making bouquets amd we cut the heads of any flowers we don't use and put them in our boiler room to dry out.  In the spring we shake out the seeds that have dried and replant them.  It is very easy to propogate new sunflower plants with last year's seeds.  A lot of our plants reseed themselves in the fields, just by the seeds falling to the ground in the fall and wintering over in the soil.  You don't need to freeze the seeds, just keep them dry and cool.  The same process is used for morning glories, which also reseed themselves, of course.