I often have people tell me they cannot get their instrument (especially the guitar) to stay in tune. However, what I am about to tell you, applies to almost all stringed instruments:
TUNE UP TO THE NOTE. What do I mean by that? I want you to start from less tension on your string and gently increase the tension until the note reaches the desired pitch.
NEVER TUNE DOWN TO THE NOTE. Stringed instrument use a guide often referred to as the nut and the saddle which are located on opposing sides of the instrument. This is a point of friction and when you induce slack, often the string sticks at this points and when the you play the instrument the tension will release leaving the note flat and out of tune.
This isn't rocket magic. Nevertheless, sometimes music teachers fail to teach the rudiments of tuning and then the student seeks a unnecessary repair. Never fear, tune up to the note. If you find things are still not in tune, take your instrument to a qualified repair person and get it fixed.
Tune up to the note! I cannot stress that enough.