It is best to handle your instrument by the neck only. The natural oils from our skin attract dust and rosin to the varnish.
Always dust the instrument and fingerboard with a soft cloth after practicing to prevent rosin dust from building up. Rosin left on the instrument can become tacky and difficult to remove.
Never use alcohol or nail polish remover (acetone) near the instrument. It will dissolve the varnish.
Never put an instrument into a car trunk. On a hot day, the varnish can bubble. Also, every dealer knows of instruments being stolen from car trunks or being severely damaged by collisions while in car trunks.
Violins & Violas
Improper attachment of shoulder rests can damage the sides and backs of violins. To avoid this, be sure the rubber on the feet of the shoulder rest is intact, and always pull the feet over the back edge, rather than sliding them into position.
When putting your violin into the case, remember to remove the shoulder rest before closing the lid. Make it a habit to automatically fasten the latches of the case whenever you close the lid, even if you don’t intend to carry it away immediately. Many instruments have suffered severe damage from falling out of a case that someone forgot to close properly before picking it up.
When removing a cello from a soft case, hold the zippers away from the body of the instrument when opening and closing them so they will not scratch the varnish. A cello bow is only safe in a soft case if the cello is also in the case, so the bow should be placed in the soft case last and should be taken out first. A bow left alone in an otherwise empty soft case is very susceptible to breakage.
Do not lean a cello up against a wall or corner. It is much safer lying on its side on the floor near a wall, but be careful not to place the instrument near baseboard heaters or heat vents.
Never leave the endpin out when the cello is lying on the floor. Others might trip over it. Do not leave a cello case standing up in a room. Passing people, shifting rugs, gusts of wind, etc. can knock the case over and damage the cello.
For more guides on string instrument care, venture here.