There are some techniques that will make your songs sound more somber. Refrain from playing in open chords. Some of the songs in this list use open chords, but it's still recommended you avoid this when writing your own sad song. If your song has chord progressions, make sure those progressions always go lower. For guitar, acoustic is always sadder than electric. However, many rock bands play sad songs with electric guitar and distortion. Again, the chords you play and the lyrics will still set the mood.