Over the years you pick up lots of little tips and tricks. An essential tuning tip is to always tune up to pitch, never down.
If you find that after tuning your guitar, one or more strings go flat after playing for a short while, it may be that you are not tuning your guitar correctly.
You will find that your guitar is more likely to stay in tune if you tune up from a note which is flat, rather than tune down from a note which is sharp. If a string is too tight – and therefore sharp – loosen it so that it goes flat and then tune up to the correct pitch. This technique reduces the chance of friction at the nut interfering with your tuning. If you loosen the string and tune down to the correct pitch you will experience problems, because the friction at the nut will retain some tension on the string. Once you start playing, the friction at the nut will be reduced and the tension in the string will loosen, causing the string to slacken between the nut and the bridge and the pitch to become flat.
You can stabilise your guitar’s tuning further by applying a little lubricant to the nut slots. Many guitarists use a little vaseline or rub a small amount of pencil lead into the slots. Specialist lubricants are also available, such as GraphitAll guitar Lube by Rene Martinez (this can be purchased from a number of outlets, including Alan Gregory Music and Musical Instruments) or Big Bends Nut Sauce (available at Gak and other outlets).