Many outdoor plants can be overwintered indoors at the end of the summer. Some can simply be potted up while others can be preserved by taking cuttings, drying the roots, or by storing the bulbs and tubers.
Plants brought in for the winter, such as Swedish ivy (Plectranthus), perilla, geranium (Pelargonium) and coleus, should be cut back by two-thirds. Store the whole pot away, watering sparingly (if at all) until early spring. A similar method can be used to store tuberous begonias. Since their bulbs are quite small, leaving them in their original pot works quite well. The key thing is to keep the plant fresh and away from humidity.
Cuttings from a streptocarpella hybrid, for example, will reward you with blooms throughout the winter months if kept in a well-lit room. Only cut off a 3″ to 6″ section from the top or branch tip of the plant at a leaf node, treat the cut with some rooting hormone, then stick it in a damp soilless mix or a jar of water.
Another way to overwinter geraniums is to dry the roots. Simply cut the plant back by two-thirds, shake the soil from the roots, and hang the plant in a cool, dark place. Some people put the plant in a brown paper bag with some fungicide and a bit of peat moss. In mid to late February, the plant will come out of its dormancy and begin to send up shoots. At that time, report the roots in some soilless mix, water, and exposed to as much light as possible.
The bulbs, tubers or rhizomes of some plants, such as those of the gladiolus, dahlia, tuberous begonia, and calla and canna lilies, are tender in cold climates and should be harvested in the fall if you want to use them again the following year. Lift these plants before frost hits (though cannas benefit from waiting until after the first hard frost). Cut off the leaves about 3″ above the bulb, remove any soil, and let the bulbs dry out for a couple of days in a shady (but not damp) spot. Dust them with fungicide and place in a paper with some wood shavings or dry peat moss. Label the bags and store them away in a cool, dry place until spring.