Read your monologue and take notes on what the stage set and props might look like if you were performing in a completely produced show.
Imagine the details of your character's atmosphere, whether it's a kitchen from the 1960s or a high-end Hollywood restaurant. Concentrate on how that environment would smell, the sounds you would hear, and the way you would dress, stand and sit as you moved within it. Let this imagined atmosphere influence your voice volume and body language as you act out the monologue.
Make notes in the margin of your written monologue whenever your character interacts with an element of the environment, so you know to incorporate the gesture into your act. Whether you mime the act of closing a window or throwing a glass of wine in someone's face, non-verbal cues are integral to a truly convincing performance.
Take a moment to mentally remove yourself from the lights of the stage when it comes time to perform your monologue. Focus on reverting to the time and place of your character. If you require a simple prop such as a chair, place it, then immediately imagine it as the type of chair in which your character would be sitting.
Create a mental map of your "set" on the empty stage. If the atmosphere calls for a busy street, for instance, decide where the curb is and don't let yourself go beyond it without looking both ways. You want to build the illusion that your character is surrounded by real, if invisible, objects.