String Instrument maintenance is necessary to keep your instrument in playing condition. It includes tasks that you can do yourself such as keeping the strings free of rosin, polishing the instrument, replacing strings, repositioning the bridge, lubricating pegs and fine tuners. There are also routine tasks that you may want to bring to a skilled technician such as positioning the soundpost, resurfacing the fingerboard, attending to the instrument’s finish, and restoring or replacing parts of the violin or its accessories that have suffered wear or damage.
Basics
Always set down the instrument string-side up.  NEVER set down your instrument on a chair, the floor, or anyplace someone may sit or walk.  Always keep your instrument and bow in its case or bag when it’s not in use.  Make sure the bow hair has been loosened before putting the bow in the proper protective compartment.  If you use a shoulder rest or shoulder pad, always remove it from your violin before putting your violin back in its case.

Temperature and Humidity

Temperature and humidity levels vary dramatically from summer to winter.   Wood, which expands in the summer months and contracts in the winter, can cause buzzing, open seams or cracks if not properly humidified.

Never expose the instrument to direct sunlight or sudden changes in temperature or humidity. Keep away from radiators or hot air vents.

Never leave an instrument in a car in hot or cold weather.

Use an instrument or case humidifier such as a Dampit when humidity drops below the normal range. Remoisten your Dampit daily whenever the heat is on in your house.

Remove rosin dust immediately after each playing. Cleaning the rosin off strings will allow them to vibrate to their full potential and can make a striking difference to the sound. Use a microfiber cleaning cloth to wipe down the strings and the body of your instrument after every playing session.

Do not use alcohol to clean your instrument. Alcohol can damage the varnish.

If the collected rosin dust is not wiped from the varnish it may fuse with the varnish and difficult to remove without damage.

Use a mild violin polish for more extensive cleaning on the body of your instrument. Cleaning the violin with furniture polish or water could damage the varnish and acoustics of the violin (water could also cause the violin seams to open).
Tuning Peg
Peg slipping or a stiff peg is a common maintenance issue and is caused by seasonal humidity variations, improperly wound strings, poor peg fit or if the instrument has not been played for some time.

Slipping pegs are common during the winter because pegs shrink when conditions are dry.

The tuning pegs may occasionally be treated with “peg dope” when they either slip too freely, causing the string to go flat or slack, or when they stick, making tuning difficult.

If pegs continue to slip or turn unevenly poor peg fit is likely the cause. A qualified technician can refit the pegs and solve the problem.

Stiff pegs are often caused by expansion due to high humidity or a lack of peg lubricant. Lubricating the peg and/or re-positioning the peg may help.
Bridge
The bridge should stand straight up at 90 degrees and not lean forward or backward. The bridge often shifts over time. If you notice it leaning then slack the strings and straighten it. The bridge is held in place by the pressure of the strings so do not loosen the strings too much.

The feet of the bridge should always be aligned with the inner notches cut in the F holes.
Strings
Many players carry replacement strings with their instruments to have a spare available in case one breaks.

Be careful that the sound post does not fall. You should not take all of the strings off your instrument at the same time.

Even before breaking, worn strings may begin to sound tired and produce an unreliable pitch. Another common problem with strings is unravelling of the metal winding.

The higher strings require replacement more frequently than the lower strings since they are lighter in construction.

A teacher can advise students how often to change strings, as it depends on how much and how seriously one plays.
Bow
Don’t touch the horsehair on your bow. The oils from your fingers can make the hair slippery so that rosin no longer sticks. At that point, you’ll have to re-hair your bow.

The tip of a bows will shatter if hit the wrong way. Be careful, it’s not a sword.

Tighten your bow before playing by gently turning the tension screw. The separation between the bow stick and hair should be about the width of a pencil. Be careful that you do not over tighten.

Loosen the bow when not in use. Keep polish and fingers away from the bow hair.

It is normal that in the course of playing hairs will get often lost from the bow, making it necessary to have it re-haired periodically.

Don’t pull or cut hairs at the very end. Leave about an inch end hanging off. If you cut the hair at the end it is likely the hair will unravel.

Dermestidae beetles (sometimes called bow mites or museum beetles) thrive in dark, dry places such as a violin case that has been closed for a long time, and feed on bow hair. If you open your violin case and notice that many of your violin bow hairs are falling off and look like they have been cut, you might have bow bugs and you probably should clean out your case.
Rosin
Before playing, apply rosin to your bow. There should be enough rosin to create friction between the bow and the string when you play.

It is not necessary to rosin your bow every time you play. If your bow has too much rosin it will produce a grainy sound and rosin dust will be visible.
Tuning Screws
If the tuning screws are tight you can rub pencil lead up and down the threads of the screws. The graphite or lead from the pencil makes them easier to turn.

Not going to play for a while
If you plan on leaving the violin for a while, loosen the strings. This saves the bridge and strings from being under too much unneeded pressure.

Tuning
Always tune from below the note, up.

Violin: The violin has four strings which are tuned in fifths. From lowest to highest is: G3, D4, A4, and E5. Tune the A string first, followed by D, G then E strings.

Viola: Tuned in fifths. From lowest to highest is: C3, G3, D4 and A4. This is one fifth below the violin, so that they have three strings in common—G, D, and A—and is one octave above the cello.

Cellos: Tuned in fifths. From lowest to highest is: C2 (two octaves below middle C), G2, D3 and A3.

Double Bass: E1, A1, D2 and G2