A good way is to use peasticks (twiggy sticks cut from hedges) and insert them into the ground around floppy perrennials so that the plants grow up through them. You can get plastic coated metal jobs that do the same thing, but I think the sticks look nicer.

The trick is to get them in place soon, so that the plants grow through them, as Sid said. If you leave them until the plants start to flop, you'll need canes and string, or special metal supports to keep the plants upright!

Look at each type of plant, too, and its growth habit - tall ones like Delphiniums need a stout stick and tie the stems in as they grow. Aconites (Monkshood) need peasticks, and so on.