Try out different topics, different ways to interpret the topic, different structures. Even when rehearsing just by yourself, try to have a coherent structure to the speech.
If you're already familiar with structure, challenge yourself by also incorporating more vivid body language, metaphors, colorful language, etc. into your improvised speech.
Want to rehearse speaking with an audience? Try a nearby Toastmasters clubMany of the topics initially in the topic database are from Loy Machedo's website: https://www.loymachedo.com. Many thanks to him for collecting the list!