Your impatiens will grow well indoors under the following conditions: Give plants average household temperatures (no lower than 60F), bright light (a few hours of sunshine will ensure blooms in the winter), keep the soil just barely moist, mist leaves occasionally, and allow the plant to become pot-bound. It will bloom more freely if the roots are cramped.
If the plant gets leggy and spindly, it needs more light. Your impatiens should be healthy and happy and ready to go outdoors in the spring time.