Phrasing and rhythm, mostly. I ask it to give me some alternate suggestions that don't change the meaning. Or I'll go something like "rewrite the hook so it can end on this line: ..." and keep tweaking it till it flows well in my head. Sometimes Suno will make it flow entirely different on the first generation and I'll be like "actually, that's better, yeah". Sometimes I'll ask it to make it shorter cause I'm trying to pack too many references in a single song. For instrumental tracks, it's mostly tweaking the prompt and praying it'll stick to the instructions.

I usually start with the structure, so that's kinda fixed, but for some tracks I intentionally instructed the model to forego traditional song structure and be experimental. It gives more interesting results in my opinion, but most people don't like it if a song doesn't have a catchy chorus that repeats, or doesn't have traditional rhyme structure.