The best practices for email subject lines include being clear and concise. You must make your subject line relevant to the content of the email. Keep it under 60 characters to ensure it displays well on mobile devices. Personalizing the subject line, such as using the recipient's name, can increase engagement. You should avoid using all caps or overly promotional language as this can be seen as spammy. Creating a sense of urgency or curiosity can encourage opens. Testing different subject lines helps you find what works best for your audience. Always aim for a subject line that grabs attention immediately.