Like I said in my post about the orchids, you do not have to worry about winter care of bonsai if you live in the tropics, but if you don't maybe you find this information useful for you.
You must become familiar with the specific varieties of bonsai you have to determine if protection (or how much protection) is required. For example the tropical bonsai candidates, such as the desert rose (Adenium obesum), can be treated as houseplants, but the vast majority of bonsai are temperate plants (not tropical) and they definitely need to be left outdoors. The most familiar bonsai specimens are hardy trees — either evergreen or deciduous. These rarely should be treated as houseplants in the winter--and then only in areas where the winters are extremely severe — because they have the same growing requirements as their full-size counterparts, and they need the same seasonal growth cycles. So leave them outside. Heel them in by burying their pots in the ground in an area where they are protected from snow cover and covered with a protective layer of mulch. If the temperatures drop well below that which the plants are hardy to, move them into an unheated garage for two or three days or into the house for up to a week or two, provided they receive adequate light and temperatures are cool (60 to 65 degrees).
The concept of wintering over a bonsai is not too much different from wintering over what is already planted in your yard. Most temperate climate trees grown as bonsai are capable of handling the severe weather as long as a few simple conditions are met.
It's better to allow them to go through a couple of light frosts at least - junipers can take anything down to 15-20 Fahr., and need to get cold to go dormant.
Remember that winter winds combined with temperatures below the mid twenties Fahrenheit (or below -5 Celsius) will freeze dry almost any bonsai.
But ting to keep temperate plants warm by placing them in the sun is a mistake. The added warmth from exposure to too much direct sun can cause a tree to wake up from dormancy too early. If this is followed by temperatures once again plunging down very low, the water in the sap can freeze within the cambium layer and kill the tree. So, please do not put an outdoor bonsai in a heated house for the winter.
Usually it is usually not necessary to take trees out of their pots. Good pots are capable of handling the temperature changes. One exception would be Trident Maple. This particular tree should be removed from its pot and the soil ball buried in the ground or a large box. If it is wintered in the pot the roots will turn to mush.
All other temperate plants can be kept in a sheltered area out of direct sun and wind. The pots should be mulched or buried to the top of the rim in the earth. Burlap barriers work well as a wind break. You can also make a wall using bales of straw.
If there is not much snow or the weather warms it is possible you will have to water your trees from time to time. Here is a good tip: once the soil freezes, don't water If the soil ball thaws and starts to dry you must water. The soil should not get saturated with water that freezing and expansion could break the pot. Be careful not to overwater in winter, especially deciduous trees as a tree with no winter foliage uses very little water.
(For more information, Please refer to my post from last year regarding Bonsai care during winter HERE)