Electronics

  One of the most common guitar electronic problems that I have encountered is the
"bad" output jack. In my experience, at least half the time this can be fixed in about ten seconds, without having to take anything apart.
  Take a one and one-half inch square piece of 400-grit wet/dry sandpaper (don't wet it), roll it into a tube (tight enough to fit inside the jack), and CAREFULLY twist it inside the jack for a few seconds, (cleaning the contacts, mostly the inside of the ground sleeve, as you twist ). When you are finished, the inside of the jack should look somewhat shiny (not black or rusty).
  This technique also has worked well for me on battery-powered effects pedals. Most of these pedals use their input jack to turn off the battery when the guitar cord is unplugged. When the jack gets too dirty, it doesn't conduct battery voltage (or guitar signal) well, and the pedal stops working. Clean the pedal's input jack the same way as with the guitar's (and, do the output jack too, while you're at it).
  Repeating this technique too often could wear out the jack prematurely (I haven't personally worn one out doing this yet), but without cleaning it at all, you may well have replaced the jack or the pedal earlier than necessary, anyway.

  Another common problem is scratchy or intermittent volume or tone controls. They can usually be fixed by spraying the inside of the pot with a spray-type tuner cleaner WITH LUBRICANT, available at "Radio Shack"-type stores. From the back side of the pot, spray a little cleaner into the pot through the gap where the three tabs are attached, turning the shaft as you spray. Some pots are sealed and cannot be sprayed. These usually need to be replaced when they get scratchy.
  In my experience, cleaning pots with a contact cleaner not containing a lubricant can cause the shaft to freeze up, and using something like WD-40 can attract and hold dirt; in my opinion, these should not be used on pots.

  Some guitars lose high end when their volume control is turned down. One way to solve this problem is to solder a .001 mfd capacitor between the input and the output of the volume pot (the two tabs that AREN'T soldered to the pot's case). This trick, borrowed from old Telecaster wiring, actually makes the sound brighter when you turn down.

  Changing the pots can also change the tone. In general, going to a higher value pot will make the sound brighter (changing from a 250k to a 500k, or from a 500k to a 1 meg), and, conversely, changing to a lower value pot can roll off high end.