Just come across The Good Ear Review US based website dedicated to publishing original monologues that show "quality writing".Worth reading and worth submitting to as well. If you haven't written one already look at a secondary character in a short story you're working on or novel-in-progress and give them a voice. You will unearth new depths to that character and perhaps a new story.
A good monologue (in my very fallible opinion) should have
A distinctive voice
Give a sense of time and place
Drip feed information
Sound like a real person
It seems to me that writing a succesful monologue you have to navigate two major challenges -- FIRST: making it feel natural even though it may not be clear who the character is addressing or why and SECOND: providing information that the reader needs to have but that the character would not realistically tell her or himself. We don't look in the bathroom mirror thinking: I am a 34 year old advertising copywriter worried about my current relationship and still jealous of my younger brother who...We look in the bathroom mirror and think: bad hair job and is that a spot? At my age?!
Click on the title of this post to go to The Good Ear Review