Imagine the person you're speaking to. If you're supposed to be speaking to the audience, imagine who they are--friends? enemies? the doorman?
Imagine where you are and the things that are around you.
Create a set by using real objects that are handy--chairs, benches, fake tree, etc.
You can use real objects for props, too--book, wallet, football, etc.
Be expressive--get mad or happy or sad or just have some attitude as you perform your monologue.
Movement & Gestures--use them. Don't just stand still while doing a monologue.
If there are stage directions—in parenthesis—in the monologue, use them or come up with your own. They're just suggestions. If a monologue has ellipses (three dots in a row), act as if you’re listening to someone else responding or speaking to you. But those ellipses might also just mean there's a slight pause. Read through the monologue and see if you're listening to someone or just pausing for effect.
Break a leg! (That's theater talk for "good luck." It's bad luck to say "good luck" in the theater so you say "Break a leg!")
DESTROY THE INVADERS
(playing an intense video game; as if talking to a parent) I can’t. I can’t leave my spot. This is the hardest section of Destroy the Invaders—which you gave me for my birthday, I remind you. I’ve got to get through Torture City before I’m captured and taken to the Dungeons of Pain. The Invaders like to hear you scream. I don’t care about food right now. I just have to keep going because there is no pausing during this section. Must not be seen. Must run from one hiding spot to the next. In this city, if they see you, they can mind control you. Weapons are useless. So, please, don’t bother me right now. I can pause the game when I reach the Screeching Ocean and board the Ship of Elephant Skeletons. Check back in 45 minutes…Ice cream? You didn’t mention ice cream. I’m coming!