I had yarrow growing in the front, and it started to get a bit overgrown and scraggly. So I employed one very good tip for perennials: Cut back after flowering. Not all perennials need this but it makes a huge difference for the ones that do. (If you want to know what perennials need what, read The Well-Tended Perennial Garden.)
It’s a simple matter of hacking back the perennial to withing a few inches of the ground. It feels a bit like you are trying to kill the plant. But don’t worry. Within a few weeks, the plant regrows. Instead of a scraggly eyesore, you will have a nicely tended perennial. In addition, it helps prevent the plant from reseeding, if that’s a problem.
In this picture, the yarrow in the foreground was cut back two weeks ago. The one behind that was cut back minutes before the picture was taken.
Now I also have my lettuce, which I purposely did not cut back, even though it is a common practice to rip it out after it starts bolting. Lettuce will usually re-seed if it is allowed to mature. I quite enjoy free lettuce.