Wikipedia sez:

The title "Martha My Dear" was inspired by McCartney's Old English Sheepdog, named Martha. McCartney has said that the song itself is probably about his longtime love interest Jane Asher. Asher broke off their engagement in mid-1968. McCartney chides her with the lyrics in the song "...when you find yourself in the thick of it, help yourself to a bit of what is all around you..." Asher inspired many of McCartney's songs, including "Here, There and Everywhere", "I'm Looking Through You", "For No One" and "We Can Work It Out". (A later "Martha" lyric explains, "You have always been my inspiration..." McCartney has also said that the song is about his "muse"—the voice in his head that tells him what words and music to write.)