Aphids and spider mites may be multiplying like crazy amidst your houseplants, especially if they are grouped close together. Isolate each plant and inspect it closely, with a magnifying glass if necessary. Do this every week or so, especially for new plants. Treat these pests by holding the plant and pot upside down and submerging the foliage in a sink full of soapy water (wrap aluminum foil over the soil to keep it from falling out). In severe cases, spray the plant with insecticidal soap following all label directions for proper and safe use.
If that geranium or bougainvillea you're overwintering inside has sent out spindly new shoots, keep trimming it back until the increased sunlight can support sturdier growth. Or, place a plant light above plants. Keep them at least a foot away from leaves so the hot lights don't burn them. Keep lights on for 14 to 16 hours a day, or, in the evening to supplement natural light from windows. An inexpensive timer from hardware or garden stores is perfect for this use.
African violets are easy to propagate by leaf cuttings. Snip off a leaf, dip the cut end in a rooting hormone powder, and stick the cutting in a pot filled with vermiculite or sand. Cover the pot with a perforated clear plastic bag and keep the soil moist. In a few weeks you'll have new plants, which you can pot up separately.
Although a sunny windowsill is an ideal spot for sun-loving houseplants, be sure the plants aren't too close to the glass or they could be damaged by the cold. Also, since heating vents are often located underneath windows, plants are prone to drying out quickly. If this is the case, place a humidifier near the plants (it will help you too!). Or, place plants on a tray of pebbles which you keep moist.