If you typically use a lot Major or Minor Triad chords in your songs, one way to spice up your songwriting would be to start adding in some Seventh chords.
Triads are the most common type of chord construction. They contain the Root of the chord, which is where the chord will get its letter name, such as G or D. And they also contain the 3rd and the 5th of the chord. You basically just pick a note in the scale to start with for the root of the chord, and then add every other note in the scale until you also have the 3rd and 5th. (The difference between major and minor chords is that for a minor chord the 3rd will be a half-step (one pitch) lower, or closer to the root)
This is very easy to conceptualize on a piano keyboard, but it is a bit tricky to visualize on the guitar fretboard. I usually explain chord construction to my guitar students using a keyboard, and then I let them know that on the guitar the theory is the same, but the fretboard patterns can be a little confusing at first.
Any of these three notes within the chord can be doubled an octave higher, and the order of the notes can be switched around. This is referred to as the particular Voicing of the chord. If you are a guitar player you have probably already realized that there is more than one way to play a G chord, by switching between open chords and bar chords, for instance. This switch changes the order and the octave doublings of the root, 3rd, and 5th, which creates a new voicing of the same chord.
Continuing in the manner above, a Seventh chord will contain the Root, 3rd, 5th, and 7th note from the starting point within a scale. The seventh will be one note before the Octave. The octave is a repeat of the Root note at exactly double the vibrations per second. It just sounds like a higher version of the same pitch, like when a guy and a girl sing the same pitch together.
For major chords there are two variations of sevenths. If the seventh is a half-step (one pitch) below the octave, the chord will be considered a Major Seventh chord, which has a somewhat pretty sound. If the seventh is a whole-step (two pitches) below the octave the chord will be considered a Dominant Seventh chord, also commonly referred to as simply a Seventh chord, which has a more dissonant sound. For minor chords all sevenths will be a whole-step below the octave, and it seems to create a more open sound, although these terms are somewhat subjective.
If you write Rock, Country, or Folk music you are most likely using a lot of Triads. You can now try changing some of the chords to Seventh chords to spice up your songs.
If you are writing a lot of R&B or Jazz you probably already have a lot of 7ths in your chords. One way to change the flavor and add variety to your chord sounds within these styles would be to reduce the chords down to Triads.
The Seventh chords in the key of C major are as follows:
Cmaj7 Dm7 Em7 Fmaj7 G7 Am7 Bm7(b5)
This order of chords is the same for all 12 major key:
Imaj7 iim7 iiim7 IVmaj7 V7 vim7 viim7(b5)
(Roman numerals are used to designate the order, upper case for major and dominant seventh chords, lower case for minor and half diminished seventh chords)
(note: the last chord is a diminished triad with the 7th added. It can either be called a half diminished 7th chord, or, more commonly, a minor seven flat 5 chord.)
If you write Blues songs you have most likely realized that the Blues utilizes Dominant Seventh chords almost exclusively on the I, IV, and V chords, turing them into I7, IV7, and V7 chords, which, in the key of C blues, would be C7, F7, and G7.
You could try turning all your blues chords into triads and playing them with a straight feel rather than a shuffle. A lot of great rock tunes have emerged from this technique. You could also try turning the I and IV chords into Major Seventh chords, which would transform your Blues Song into a Major key chord progression. (For instance, in the key of C Major, I, IV, and V chords would be Cmaj7, Fmaj7, and G7, respectively, but in C Blues they would all be Dominant Seventh chords, C7, F7, and G7).
So add a few sevenths, and take a few a way, open up some new sound palates for your songs, and you will quickly find yourself in Seventh Chord Heaven.