Mixers get damaged and worn out over time due to the DJ using them, and eventually will need service to keep them going. Many parts of the mixer can be serviced, and this page is a summary of the techniques that have been tried and which are recommended by the top DJs. Skip to the section about the component you wish to find out about servicing.
The crossfader is a somewhat delicate part of the mixer and after a long period of use it may crackle or have a bad cutoff. Most crossfaders can be replaced, but the technique to do this varies between them. Instead, cleaning the crossfader is a good idea!
Some people/websites recommend using WD40 for cleaning the crossfader of dust and particles... DO NOT USE WD40! It may work for a few minutes/days/hours after having put it in, but in the end it will go sticky and not only restrict movement in some way, but also attract more dust! Instead, you want to use electrical switch cleaning fluid, available from hardware/electrical stores. Alternatively, some DJ stores/sites stock specialist crossfader cleaning fluid (re-packaged switch cleaning fluid)... go with this if you are not so sure.