In late-May to mid-June, cut back to a pair of strong lateral buds the growing tips of perennials such as fuchsias, dahlias and penstemon and annuals like sunflowers and cosmos. This will encourage the plants to bulk out and produce a stockier plant with more flowers. With sunflowers and cosmos, you'll have smaller flowers (called prince flowers) but more of them, rather than a single shoot with one large flower (called the king flower). Small auxiliary buds will develop within a week or two. It seems a drastic measure, but the flowers should only be delayed by a week or so.
Use the clippings of perennials as softwood cuttings. Remove the bottom leaves from the stems of the fuchsia and dahlia cuttings, pinch out the growing tip and push 3-4 of them into a small pot, containing some gritty potting compost (50:50 mix of grit and compost), water gently* and put in a propagator or add some small canes and cover with a clear plastic bag. Put them somewhere cool but bright (a greenhouse or conservatory is probably a bit too warm for them). They should have rooted in 3 to 4 weeks, ready to pot on into individual pots, or into the garden.
*Use tap water rather than water from a butt as this can introduce infection.