Bonsai trees require lots of attention and specialised care in order to survive. Over- or under-watering, and too much or too little sunlight or heat, are all factors that can contribute to a bonsai tree becoming sick or dying. If your bonsai tree appears as though it won't survive, you may be inadvertently doing something wrong.
Sterilise your pruning shears with wood alcohol so that when you prune away the unhealthy foliage, you don't spread any bacteria to the healthy branches of your bonsai tree.
Cut away any dead or dying branches or stems. Do not cut away any healthy portions of the plant.
Remove your bonsai plant completely from its pot and remove any soil clinging to the roots. Carefully prune away some of the roots, as bonsai roots can quickly grow too big for their pot and become cramped and compacted.
Re-pot your bonsai tree in a bigger container. Before doing so, sterilise the inside of the container with wood alcohol. Mix sand with the soil you use, as this assists bonsai roots in draining. Make sure the pot you use has enough holes for draining; if bonsai roots do not drain properly, it contributes to the sickness or death of the plant.
Situate your bonsai plant properly. Bonsai trees need between three to six hours of direct sunlight daily, or they can begin to have a lifeless appearance. If there is nowhere indoors to put your tree that will grant it enough access to sunlight, put your plant outside during the brightest hours of the day.
Water your bonsai tree properly. The soil should be kept moist but not wet; watering once a day until water drains through the holes in the pot should achieve this effect. If you find the soil is always dry to the touch after once per day watering, water the plant twice a day and monitor it closely. Once it begins to retain moisture, go back to watering once per day. Make sure you use a clean spray bottle to water the plant. Not only does the soil need daily watering, the branches and foliage need to be misted once per day as well. Make sure the water temperature is mild -- not too hot, and not too cold. Also, water the plant during the cooler parts of the day.