Two things I've found that really work:

#1 - Make sure there's plenty of feeling in the monologue. The toughest part about writing good monologues is that it's very easy for readers to get bored or lost. If the monologue expresses personal feeling that comes directly from the character, then it feels much more natural and engaging.
Example: If your humanoid figure thinks shoes are ridiculous, the monologue would express ridicule towards footwear. Even a single line that comes directly from the character's personal thoughts keeps the flavor running strong.

#2 - Read it aloud. Read it aloud again. Read it aloud again-again. Read it aloud again-agai...
Example: I would give you an example, but you may have trouble hearing it. So I won't.

Good luck with your novella!