Sundew care differs depending on the origin of the plant. Most simply, they can be divided into temperate and tropical varieties, although some plants may have more nuanced care. Temperate species, such as D. rotundifolia, filiformis, intermedia, and anglica, have cultivation guidelines almost identical to those of Venus flytraps and North American pitcher plants. To be safe, look up the species of plant that you have - a good resource is this page.
Where to Grow: Temperate species should be grown outdoors. Tropical species are best kept indoors.
Light: Temperate species should be grown outdoors in full sun. Tropical species should be grown indoors under very bright lights (avoid incandescent bulbs, which give off too much heat). If the sundew is lacking in dew, this is a sign of too little light.
Water: Use distilled/rainwater/reverse osmosis water ONLY. That means NO TAP WATER. Place the plant in a tray of water about 1 inch deep at all times.
Soil: Use a 50:50 mix of sphagnum peat moss and coarse (#12 silica sand or similar) sand. If sand cannot be found, perlite can also be substituted. Make sure all soil ingredients are free of fertilizers and added minerals.
Containers: Plastic pots are best. Avoid unglazed terra cotta plants, because they can wick water away from the plants and also leach minerals into the soil.
Feeding: The plants will likely catch all the insects they need, but can also be fed small insects from time to time. Use insects that are not much bigger than the leaves themselves.
Fertilizing: As a general rule, avoid using any fertilizer on sundews.
Humidity: Usually not a concern. Low levels of dew on the plants is a sign of low light, not low humidity.
Dormancy (Winter Care) - Temperate Species Only: Temperate sundews will go into dormancy in the winter months, generally between October and March. During this time, they may stop growing and look dead. Some species will produce a hibernaculum, which resembles a rosette of tightly packed, short leaves. Dormancy is triggered by shorter daylight hours and colder temperatures. In the mid-Atlantic region, plants can overwinter outdoors provided they are either mulched heavily with pine needles/bark (bury the pots under several inches of mulch) or placed into a cold frame. Alternatively, some growers have had success with placing the plants in an unheated garage as long as the plants can still receive some light. During dormancy, the soil should be kept damp, not very wet. The plants can survive frosts and even some brief freezes.