Use different chords from the rest of the song. Create a different feeling for your bridge by using the related minors (or majors, if your song is in a minor key) to the song's chords. The related minors are always the musical sixth, second and third. Number the notes in the key from one to seven. For example, in the key of C major, C is the first, D is the second, E is the third, F is the fourth, G is the fifth, A is the sixth and B is the seventh. Therefore, if your song uses C, F and G, you can write a bridge around the chords A minor, D minor and E minor.
Change the rhythm. If the song you are playing has fast, punk-style strumming, you can use the same chords but change the rhythm you play them in. Strum chords and allow them to ring out. Many rock songs use "power chords," and these can sound particularly effective when allowed to ring out for a bridge. Alternatively, pick the notes of the chords individually to create a mellow bridge.
Use higher notes for the bridge. This can be done simply on guitar by using "octave slides" at high frets. For example, if your song is primarily composed of the A, D and E power chords --- A played with an open A string and the second frets of the D and G strings; D at the fifth fret of the A string and the seventh fret of the D and G strings; E at the seventh fret of the A string and the ninth fret of the D and G strings --- move each chord 12 frets up, and only play the notes on the A and G strings. These are the same notes an octave apart, so the song's notes are the same but much higher and more focused.
Determine which elements of the song you need to bridge together. For example, the verse and chorus could sound too similar and need something to split them up in order to give the chorus prominence. Likewise, the verse and chorus could sound too different and require something to make them seem like part of the same song.
Use different chords or a different melody if the verse and chorus sound too similar. For example, a simple chord structure of C, F and G in the verse and chorus can be broken up by switching to a different chord pattern, such as D then F, in the bridge. Although this isn't strictly in key, it adds a different dimension to the song and accentuates the shift to the chorus. You can also use suspended chords or seventh chords in this type of bridge.
Introduce the new elements gradually if the bridge and chorus sound too different. For example, if you have a thumping riff backing up the verse and a quieter chorus, you can deconstruct the riff in the bridge. Remove superfluous elements such as fills, and focus on the most important chords to slow the pace. To go from quiet to loud, try palm-muting the chords for the chorus and slowly taking your palm away to increase the volume.