Any string instrument made of wood needs humidity control to remain stable. Generally the humidity should be between 35% - 60%. I have often heard museums cite a humidity of 50% as their goal. I generally keep the humidity a bit lower than that because we have a very dry season in Vermont. Sudden changes in temperature and humidity are particularly bad for the instruments, so I keep the minimum humidity around 35% - 40%. With such dry weather in the winter a player will often be playing in spaces that are not humidity controlled. If the instrument were kept at 50% humidity, and then taken to a performance for a few hours at 15%-20% humidity it would be a bit more of a shock for the instrument, so we try to keep them stable, but avoid the big swings. In the humid summer months I do not dehumidify, so the humidity is somewhere around 50%-60% during that period with occasional peaks closer to 70% when it rains for a while.
Controlling the humidity is done in a couple different ways. You can control the humidity in the room where the instrument is kept or keep control of humidity in the case, and always put the instrument back in the case when you are done playing. Even if you keep humidity in the room controlled I would still keep a humidifier in the case for travel in dry months. Some humidifiers are able to be attached to the inside of a case by either velcro or some sort of clip. For cellos or basses in soft cases I strap a humidifier to the back of the neck with some padding between. It is important to make the humidifier is not leaking or still damp on the outside when placing it inside the case. The Dampit humidifiers are notorious for dripping inside an instrument and causing damage or open seams.
Most humidifiers are essentially a sponge inside a plastic case. The sponge needs to be rung out really well so it doesn't drip, but that means there isn't much water left to evaporate. You will then need to refill it more frequently. I use a humidifier that has a water reservoir vacuum sealed with a wick to allow evaporation. It can usually go a couple weeks between refills.
To track the humidity accurately you should use a hygrometer. They generally need to be calibrated occasionally, so it is good to have one that can be adjusted. I have tried many dial hygrometers that come with some cases. I have yet to find one that seems to work well inside a case. I have re-calibrated them, and still found that the needle doesn't move much. They will often stay at 40% even when I leave them in a room with much lower humidity. You may want to try a digital hygrometer. David Burgess has some good information on the subject as he has tested many hygrometers. www.burgessviolins.com/humidity.html
One final note on humidity control. I have begun using weight as a relative measure of moisture change in the instrument. If you really want to know how much the instrument is changing in moisture content you can track weight over time. It has been an interesting metric for me, and it has changed the way that I think about the seasonal swings of the instrument. I've seen an instrument gain as much as 1% of it's overall weight in just a couple days of rain in the spring. That's a big change in moisture content in a short period of time. I expect that I'll write more about my weight tracking and data collection in the future as I continue my study.