I,too,have heard the "shut them in the trunk for at least a week in the summer thing-- but NO PEEKING!!" This assumes sunny,hot,dry days,which have been in short supply lately!!I cut them when they start to feel a little "papery" and hang them upside down in bunches in a dark closet that has an airconditioning vent ( I imagine a dark room would work,too)They dry beautifully,AND hold their color!!!I strip the leaves off the stem before I dry them. Don't try the attic, it is way too hot. The slow drying seems to help them retain their color. They will be in shades of blue and sort of mauve. We used them to decorate at my daughters wedding several years ago.If a few blooms look bad, cut the individual brown floret out with manicure or embriodery scissors.

Joe pye weed may be blooming then along with iron weed but anything you dry in the meantime can be spray painted a different color. You might try making your own creations from something cheap and plentiful like sweetgum balls and pine cones. You can mix silk flowers from craft stores with real ones. My sister makes hyacinths out of those plastic 6 pack holders, puts them in her yard and you can't tell they are not real. Go over to the craft forum and pose your question. Friends can often put you on the spot to come up with something pretty on an exact date in a specific color and for free. Insist your daughter help, as it is her friend.

How about asters? There are lots of white, blue and purple cultivars.
Also, think about planting lilies for succession planting. I recently purchased both purple and white lilies and I plan on planting them in a week or two. Imagine, lilies blooming in August! You can do the same thing with gladiolas.

GGG

Roses dry well also. I hang them upside down in the garage for a couple of weeks and they retain their color fairly well. Another thought might be contorted filbert, or contorted willow branches. They really add lots of interest to flower arrangements.