You've probably heard the advice that when designing a garden you should use plants of various heights. You've also read that you should arrange them to avoid the "school picture" look (all the tall ones in the back and short ones in the front)—though that arrangement seems the simplest way to make sure everything gets seen. Here are some tricks to mixing up plant heights successfully:
Consider the plant's form. Two plants described as "5 feet tall" can be very different. One may be a dense mass of foliage and flowers. The other may be a low clump of leaves topped by airy 5-foot-tall flower stalks, such as a daylily (Hemerocallis; shown). The latter will work better toward the front of the arrangement.
Consider the plant's weight. Some tall perennials are very willowy, with thin stems. They sway in the slightest breeze and their delicate stems make them almost transparent. Russian sage (Perovskia atriplicifolia) is a good example. These can work toward the front of the garden. They only thinly veil plants behind them, and they offer compelling glimpses as they move in the breeze.
As for short plants, consider their overall shape, and reserve the most frontward positions for those with neatly rounded/mounded habits. Cushiony plants do a fine job of softening the edges of a bed or border.
Plan your garden with the Perennial Garden Wheel, a 3-D tool for less than $8.
Get illustrated gardening how-to articles for under $5 with Horticulture's Smart Gardening Techniques downloads