If you are the sort of gardener who loves seasonal flowers and enjoys watching the poppies, petunias, pansies, hollyhocks and what have you bloomed every year, then chances are you also want to be able to see the same excellent results every year.
Also it makes sense that if you buy flower seeds one year, you should not have to keep repeating that expense every year. With a little care, you can use the seeds from the flowers that bloom this year, so that you have a similar riot of color next year.
Many gardeners advise that the flowers should be permitted to wilt and dry on the stem rather than be chopped off for dead heads. When the flowering season is over you can clear the area of your seasonal shrubs and let them dry in the sun.
Since you have not cut off the flowers from the stem, the seeds in them have had a chance to fully mature and once fully dry, it becomes quite easy to get the seeds out: the dried head of the poppy once the petals have fallen off will swell with seed, the petunia petals, when they fall off will also yield their seed easily which look much like mustard seeds, and so on.
Once you have all the seeds fully dry and segregated in the manner that you want, you can store them in ziplock bags, label them and put them away in a cool dark place, until they are ready to be planted again!