Lead with something new: a recent development, a question about their situation, or a shorter version of your original ask. If your first email was long, the follow-up should be two or three sentences. If your first email made a big ask, the follow-up can scale it back: "Happy to start with a 15-minute call instead of a full demo." That reduction in friction often produces a reply where the original ask didn't.
The subject line can be a reply to your original thread (which keeps prior context visible) or a fresh line if you're changing your approach. Reply to the thread when the core ask is the same and you're just nudging. Use a fresh line when you're leading with something new: a different hook, a scaled-back ask, or a new angle entirely.