When cosmos seeds are ready (upper left) the flower goes all spiky and brown. The seeds are so thin, you have to be careful not to break them as you gently pull them off the head.
Morning glories, on the other hand, make seed pods that look chubby and pregnant (upper right). Eventually the pods dry, leaving tiny, sturdy, black seeds.
Milkweed is, again, completely different (lower left). It makes long skinny pods, that I haven't gotten in sync with yet, timing wise. Supposedly, you should harvest the seeds within, when the pod just starts to split. If you force the pod to split open, the seeds inside will be white and unripe. If you wait to let the pod completely open on its own, the seeds--carried by white, fuzzy tufts of fluff--will all blow away before you can capture them. So far mine have all opened up without me!
Then there are Sunflowers, a plant that is entirely obvious about its baby making (lower right). When you start seeing seeds, just snip the head and leave it somewhere to dry until the seeds come off easily.
In addition to saving flower seeds, I'm also saving some herb seeds--a great way to get a bonus from your plants. I have dill and cilantro right now, and the seeds of both are easy to spot. Once about a third of the seeds on the plant start to look dry, you can cut down the whole plant and pick off the seeds. If you lay them out on a tray or paper towel, the seeds that are still green will turn dry and brown within a few days to make super-fresh spices.
Does anyone else save seeds? What kinds have you saved and how did you do it?