Special note concerning humidity:

Be aware that very low humidity (less than 30%) will have an effect on your instrument.  Wood shrinks as moisture moves from it to dry air.  Depending on many factors, wood can crack to relieve the stresses imposed by moving wood and non-moving glued seams.  To protect your instrument, invest in some kind of humidifier.

You can use a dampit
which is essentially a sponge in a tube out of which water slowly evaporates, keeping your instrument happy with moisture.   A dampit is put inside the instrument through the F-hole - the black disk on one end (in the right of this picture) prevents the dampit from falling completely into the body.  They are normally used only when you're not playing.

You can also use any one of a number of "in case" humifidiers.  One type is a ceramic cylinder that, once soaked in water, slowly releases moisture.  This type sits in your case.

Whole-house humidifiers benefit not just your instrument but everything else in your house that's sensitive to low humidity - including you!  At Tulsa Violin, we use the older model equivalent to these Kenmore humidifiers, available from Sears.  With these, we keep the Shop's relative humidity no lower than 40%, even during the driest stretches of winter.

If your budget doesn't have room for any of these options, we have the following suggestion:  Buy the kind of soapdish that has two pieces that lock together, making a little plastic box.  Drill a number of holes on one side to ventilate the soapdish.  Soak and wring out a sponge, put it inside the soapdish and secure the two pieces of the soapdish together.  This gives you low-dollar but effective "in-case" humidifier that works just the same way as a dampit.