A secure cage is vitally important - king snakes are notorious for testing their enclosures and escaping from the smallest of spaces. Any enclosure requires a secure, latched top, and while it may be more expensive to set up a cage like this it will be worth it. Keep in mind that snakes can get through spaces that are so small it doesn't seem possible. Placing plastic tubing (like aquarium air line tubing) between the rim of the tank and the lid may also help prevent escapes.
King and milk snakes should be kept one to cage/enclosure. These snakes will not hesitate to make a meal out of a cage mate.
While hatchlings can be started out in a small cage (e.g. 10 gallon tank), medium sized snakes need a 20 gallon tank and full grown snakes will need an even larger enclosure, such as a 60 gallon tank. King and milk snakes are quite active and need the room. Giving them room to stretch out is also thought to reduce the incidence of respiratory infections.
For a substrate in the tank, a variety of materials can be used. For new snakes, paper towels or butcher paper are ideal to facilitate cleaning allow monitoring of feces until certain the snake is free of parasites. Various substrates that can be used include indoor-outdoor carpeting, reptile bark, mulch, or aspen shavings (never use cedar, redwood or pine). If shavings are used, it is important to make sure it isn't ingested with the snake's food. Indoor-outdoor carpeting (e.g. Astroturf) is nice in that you can have two or more pieces ready cut for the cage, and can just remove the dirty flooring, replace it with a clean one (then clean the soiled piece for the next time the cage needs cleaning). Whatever is used, cleanliness is very important so choose something that you will be able to clean as often as necessary.
Several hides should be provided: half rounds of bark, commercial rock hides, overturned flower pots, half coconut shells, and even cardboard boxes can be used for hides. Other cage furnishings can include a selection of rocks and branches.
Temperature and Humidity
As with other reptiles, providing the appropriate heat gradients is of utmost importance to the health and well being of your snake. Generally, milk and king snakes need a gradient of about 76-86 F (24-30 C) in their cage during the day, with a drop to about 70-74 F (21-23 C) at night (there may be more exact temperatures for different species). The gradient should be horizontal as well as vertical, and hides should be provided at least at each end of the gradient.
Most owners prefer under tank heaters (place under half the tank) to provide the heat. Under no circumstances should hot rocks be used. If overhead heating used, radiant heat sources (e.g. ceramic elements, available at pet stores) are preferred to incandescent bulbs especially for nocturnal species.
For humidity, providing a shallow dish of water in the cage should be sufficient. Since snakes will often defecate in the water it should be cleaned out daily. King and milk snakes do not need high humidity levels - 40-60% is sufficient, although sometimes during shedding they may benefit from added humidity. If your snake is having a hard time with a shed, try misting the cage lightly, or provide a humidity box (take a covered plastic container, cut a hole in the lid just large enough for the snake to climb in, and line with moistened sphagnum moss).
King and milk snakes are fed mice or baby rats. As a general rule, feed the snake the size of mouse that is roughly equal to the width of the snake at its widest part (excluding the head). Young snakes (sub adult) should be fed twice a week. Adults can be fed adult mice (or weanling rats) once or twice a week. Start with once a week, and if the snake is too lean (body not rounded, can see ribs or back bone) feed twice a week. Many king and milk snakes tend eat less in the fall and winter (okay as long as snake stays in good body condition).
As with other snakes, feeding pre killed mice is recommended, to ensure that the prey cannot injure the snake.
After giving a new snake a couple of days to settle in, you can start handling your snake. Be gentle and persistent, with daily short sessions at first to build trust. At first the snake will probably try to get away, and may even excrete a musky scent from its anal glands (smelly but not harmful). It shouldn't take too long for the snake to get comfortable and settle on your hand/arm. Remember these snakes are constrictors so they may try to wrap themselves around your arm (also not dangerous) - unwrap them from the tail end (their head end tends to be stronger).
Regurgitation is relatively common in these snakes, and can result from handling them too soon after a meal. Other causes can be food that is too large, an enclosure that is too cool, or illness. If regurgitation is recurring after correcting the environmental and feeding causes, check with a vet.