You don't necessarily have to buy new coleus plants each year. I tend to buy a couple six packs but also supplement our coleus plants in other ways. Coleus is a very easy plant to propagate so I take cuttings before the frosts come in the fall to root and bring indoors. It grows very nicely as a houseplant! In the spring I take cuttings from the plants I have indoors and plant the potted ones outdoors to grow outside.
I also allow my coleus plants to flower and eventually produce seed. Most gardeners try to avoid the flowers for a while by pinching back the stem tips to make bushy plants. I'll pinch them back for a while but once they start to flower I let them go so that the coleus produces seed. Then I collect the seed and sow it in the spring. Planting from seed is a very cost effective way to grow your garden - especially when you grow your own seed!
Sometimes they even sow themselves in your garden!
Do you grow coleus in your garden?