Are there any traditional ideas on what key to change to and also how to do it, to get a certain effect or make it sound good? I know the general rule is that everything works as long as it sounds good but i also know that there are often typical strategies and techniques frequently used in pop, cinema scoring, jazz etc...
There are two broad categories. Modulation to closely related keys - 'keeping it in the family'. And modulation to more distant keys for contrast.
The first kind include modulation to the dominant, subdominant and relative minor/major. Add a sharp, add a flat or just choose a different tonal centre for the same scale.
The second kind include - well, everything else!
A 'Truck driver modulation' up a half step or full step for the last chorus of a pop song. Well-used, but still effective.
A 'Chromatic mediant' shift - C major to Eb major or C major to Ab major. A lovely warm modulation, stock-in-trade of the Golden Age songsmiths, but used in all types of music. A strong pivot note is the unifying feature, C to Eb has G, the dominant of C, in common, C to Ab has C, the original tonic. (An alternative excuse is 'relative major of the parallel minor'.)
C major to E major is also effective, bright rather than warm. Some say it sounds like 'the sun coming out'. I can relate to that. Can you? The note E is common to both tonic triads.
C major to Bb major (or G minor) is harder to dress up with a plausible functional justification, but it's used and it sounds good. Laid-back.
C to F# is IMPOSSIBLE to justify, other than 'as far as you can get'! But that can be useful. Sometimes we need to portray conflict.
That's most of them, isn't it. What's left? Down a half-step - C major to B major - is probably most likely to be needed when tailoring an arrangement to a particular singer's 'money notes'. It can be done.
How? Lots of ways.Here;s a few:
You can get 'most anywhere with a ii7 - V7 - I into the new key. Even if the ii7 is quite foreign to the old key. Or you can 'just jump in'. A pivot note is useful - linger on any note from the old key, re-harmonise it as the leading note in the new one. Or prepare the way with some chromatic modifications - for instance, if you're in C major and want to get to Eb major, start throwing in some Fm chords - subdominant minor is a very common 'nearly diatonic' chord. That will make a later Fm7 - Bb7 - Eb sequence sound much more inevitable!
Are there any traditional ideas on what key to change to...
Yes. The standard changes are to "close" keys which are keys with key signatures differing by only one sharp or flat, or to keys differing by mode.
Close keys are the
- relative minor from major or relative major from minor
"Parallel keys" are those differing by mode but sharing the same tonic
- going to parallel minor from major
- going to parallel major from minor
...and also how to do it,
The really common way is through a "pivot" chord, a chord found in both keys, but many times music will simply play the dominant of the new key to make the transition. There are other ways to change key.
...to get a certain effect or make it sound good?
There is no list of moods. In really simplistic terms mode changes major to minor and vice versa can have a cheery to sad mood, or dramatic to tranquil, and so on, but it's very subjective.
A big part of key changes is structural. Key changes often define sections of music. Using several key changes is a way to build a bigger structure and create longer compositions.
There are some fairly standard key change orders in classical style. Major key works often modulate to the dominant first. Minor key work often modulate to the relative major first. Modulating to the subdominant can create a denouement feel and so is often found near the end of a work.
Keep in mind the effect of a key change is not entirely about the harmonic method used to make the change, but just as much determined by factors like rhythm and phrasing.
For example, in the interior sections of a piece you will often find sequential harmony which moves through several keys with fairly unbroken rhythm and that will have a smooth, flowing effect. By comparison, when a sonata beginning modulates from tonic to dominant there is often a rhythmic break that sort of announces the transition and provides a break before a new level of activity. The harmonic means to both changes will often be the same: pivot chord, dominant, new tonic. But, the rhythmic treatment differs and that creates the different effect.
Here's one for you, and it's not founded so much in music theory as in practical application.
There are some songs that are in a good key for one of your band's singers. That key works for them, and it suits the timbre of their voice.
Sometimes, it's quite a dramatic effect to modulate (at mid-song, after a chorus or two) up to another key that better suits one of the other singers in your band. Let them take over the singing duties at that point, and have everyone else chime in on the last Chorus with appropriately higher (and louder) harmonies.
It's a real crowd pleaser, trust me!