Modulation

From Young Composers

Key-changing, or modulation, is an incredibly effective tool if used well. When you've been in one key long enough, you should think about moving out into new ground. This is not always necessary or appropriate; some excellent pieces stay in the same key the entire time.

When changing keys, decide how you will approach the shift. Do you want to give warning, or just change gears surprisingly? If you want to give warning, try a leading-tone or dominant of the new key, with perhaps one or two tones changed to fit the old key a little bit. If not, what I like to do is use the melody note I ended on in the old key for the root in the new key, or even just the beginning note of the melody (which will imply a certain key by diatonic structure... unless you want to change your melody modally).

In general, rhythm should change according to the activity level of your piece. Don't have the rhythm change every measure if you're writing something very structured or slow. Of course, you could change something small without much impact. In a piece more spontaneous and wild, you might want to change dramatically every few measures, even throw the melody to the other hand and have the former melody hand take a new rhythm for the accompaniment.

Don't be afraid to write a measure with a simple motive that gets a little lost. A 'gap' measure often can be very useful to create a logical break between melodic sections. Also, for a transition to a new theme, you might think of inserting one measure of a related meter, like 2/4 in 4/4. It should extend what you were doing in the previous 4/4 measure, creating tension that will be resolved when the new theme is heard.