Act as a seasoned email marketing specialist with 10+ years of experience in the beauty industry. Craft a detailed guide on the best practices for email marketing tailored specifically for hair salons. Cover the following key areas: [subject line strategies to increase open rates], [content ideas that engage and retain clients (e.g., promotions, styling tips, seasonal trends)], and [optimal sending frequency and timing for maximum engagement]. Also, include tips on [segmenting email lists based on client preferences or purchase history] to personalize campaigns. Provide actionable examples and avoid generic advice. Focus on strategies that have proven successful for small to medium-sized salons looking to boost bookings and loyalty.