My domain shows up on a spam blacklist, how do I get delisted?

First step is understanding why you got listed — Spamhaus and most blacklists list for a reason (high complaint rate, sending to spam traps, compromised sending, etc.), and delisting without fixing the underlying cause usually just gets you relisted quickly. Can you tell me roughly when your bounce rate or complaint rate started climbing, and whether your contact list includes any purchased or scraped addresses rather than people who explicitly opted in?

Yeah, that's almost certainly the cause — purchased lists are one of the most reliable ways to hit spam traps, which is exactly what triggers Spamhaus listings. To fix this: remove the purchased list from your sending entirely, then follow Spamhaus's delisting process for your specific listing (usually a form at spamhaus.org confirming you've addressed the cause). Going forward, stick to opt-in contacts only — there's really no reliable way to safely send cold volume to a purchased list without risking this again.