Stake taller plants before they get too big and flop over sideways, ruining your display and possibly flattening nearby flowers too. Delphiniums, peonies, dahlias and oriental poppies are all prone to toppling over sideways, especially after heavy wind and rain and sometimes under the weight of their own flowers.
Earlier is always better, as with supports in place before they get too tall plants happily grow out and around to cover unsightly posts. Strapping a plant up after it's already fallen over inevitably leads to a 'trussed' appearance, so anticipate any imminent collapse as soon as you can to keep things looking natural.
A simple solution is circles of bamboo cane or short hazel sticks, poked into the ground around your clump. Loop twine around each cane to make horizontal strings at 30cm intervals up the canes.
Brushwood or twiggy prunings of hazel or birch poked in among your clumps and woven together make unobtrusive stakes which quickly disappear among the foliage while still holding wayward stems in place. Look out for bundles of peasticks in your favourite garden centre. You'll also find sturdy wire or rusted-iron hoop plant supports: poke the ends into the ground and the hoop will restrain any outward tendencies nicely.