I posted this reply on Chopinfiles not too long ago.
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Having a properly tuned piano is very important, not just of the piano, but of how you play. If you play on a poorly tuned piano, you may develop poor pedal technique to try to "cover" up the deficiencies of sound.
How often:
It depends on numerous factors. The most important factor is climatic: temperature and humidity. If you live in a region that often fluctuates throughout the year or even week to week, then to keep a well-tuned piano it should be tuned very often, as often as four times a year if not more. But if you live in a region with a very stable climate that fluctuates very little, then it may only be necessary to have it tuned once a year or even less.
An example of how a $75 tuning got ruined in less than two weeks. This happened to my piano. It was moderate temperature when it was tuned. However, this was during the beginning of spring season. Less than two weeks later, there were two consecutive very hot days followed by some cool days. On the first hot day, all of the notes' pitches dropped. It was very audible. Second day just made it worse. $75 down the drain? Oh well, this was the pianos third tuning in over 10 years.
Because of the climate here in San Francisco, depending on how much time I have, I tune it often though not all the notes at a time. Some unisons (the two or three strings that make up the note) go out of tune with each other so I have to tune those strings back into tune with each other. This is a quick fix so notes sound "pure".
How much it costs:
It usually costs anywhere between $50-75 dollars. Certain tuners may charge as much as $100. You should ask someone for a recomendation for a tuner and also ask how satisfied they were with the tuning.
Keep in mind that tuning piano's is different with different pianos. Certain piano's usually won't hold pitch very long while others hold it very well. Usually the cheap or very old pianos with worn out pin blocks or poorly maintained ones don't hold pitch. And the condition of the strings also affects pitch. If strings are rusted, then the strings will not vibrate un-impeded and tuning will be dificult as the string won't vibrate at a certain pitch when tuned. So if someone has negative feedback about a tuner, first take a look at their piano. If it's old and poorly maintained, then there is little a tuner can do but patch the piano up. So ask someone with a well-maintained piano for a tuner.
Can you tune it yourself?
Yes, you can. I tune my piano myself. If you have a lot of patience, then tuning your piano may be a worthwhile venture. You need proper tools to tune the piano.
You need:
tuning hammer ($20-$100)
mutes and/or strip mutes ($5-$20)
reference pitch (tuning forks [$8-30] or electronic tuning device[$40-$200+])
book on tuning ($10-$20)
proper hearing (priceless)
The learning takes a huge amount of time. Learning how to 'tap' the hammer to turn the pins is the most important initial aspect. The most difficult aspect of tuning is learning how to tune the notes to proper pitch with just one reference note. However, with modern technology, electronic tuning devices are much more convenient as it read out the pitch of each note so one does not have to rely on a highly skilled ear to tune. Most tuners rely on an ETD. It's cheating but you don't have to develop aural skill to tune the scales. I use an ETD and wouldn't be able to tune the piano without it. I'm a cheater but I'm going to learn to tune without it someday.
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It's really not that hard to tune your piano and you are not disassembling anything. You're just turning the pins very very very very slightly. And there isn't much reading involved either. There is very little theory that is in the act of tuning. Tightening the string will make the string vibrate faster; loosening it will make the string vibrate slower. That's all the theory you need. The rest is done with your ear to tune the other notes to proper pitch (very difficult). This is where an ETD can be of very good use as it will read out how far flat or sharp a note is so you can tune it to pitch.
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