Use language creatively. Write your monologue through the use of great language, not great grammar. Although grammar is an important aspect of writing, do not think that it makes your writing interesting. The use of grammatically correct sentences has little to do with the power of your monologue. A good monologue stands out, and this is through a character's ability to use language creatively and beautifully, to be able to twist and bend phrases to his will. Know your characters specifically, and create distinct differences in the language they use.
Mix emotions into the monologue. The most important thing to know is that there are only four emotions: happiness, sadness, fear and anger. All other feelings are made up of a combination of these four. For example, fear and sadness might be despair. It is important to keep this in mind, as it will allow you to send the monologue in any direction you wish it to go, keeping the reader gripped. A monologue that may at first be angry has the ability to travel through fear and sadness and end up in happiness. By developing the ability to twist these different emotions together in your monologue, you intrigue the reader.
Know your audience. This is often deemed the most important aspect of a good monologue. The "audience" is not the group of people reading your novel, but the group of people who you are specifically trying to reach out to. Knowing the people your character is speaking to will enhance the effectiveness of your monologue. This will give your character a recognizable voice and make your monologue stand out.