This article is from the Classical Guitar Playing FAQ, by Joshua Weage (firstname.lastname@example.org) with numerous contributions by others.
Invest in a metronome. Also used for tremolo (Q3.3), a metronome
offers a regular tock-tock-tock... at a rate easily dictated by
the user. Here is how to use it.
Say you wish to play a scale faster. Set the metronome going at a speed
you can easily play along to. It is often better to play two, three or
four notes to each tock of the metronome. Ensure you can play
the scale at this speed exactly right before proceeding.
Push the weight on the metronome down by two notches. Play along at
this faster speed, maintaining correct fingering, alternation, even
volume etc. When the scale is mastered at this speed, increase the
metronome speed by another two notches and play again.
If at any time you feel the metronome is going too fast, put it
back by ONE notch. Hopefully, you should be able to manage at
Another aid to increasing speed is play notes in pairs, one
as a dotted quaver, the other as a semiquaver. So, if you are
playing a scale of C, play the C as a dotted quaver (count 1 e-and-)
and then the D as a semiquaver (count (-a-), then the E on the
next beat (count -2-...). You can also of course play
the semiquaver first, and then the dotted note. So you'll
get a dump-e-dump-e-dump-e-dump-e rhythm. Again, use the
When increasing speed it is important not to forget your technique. Its not
too hard to play fast, but to play fast well, accurately and with tone
control is another thing that takes patience and practise to perfect.