Common mistakes when starting a new fitness regimen include setting unrealistic goals, which can lead to disappointment and burnout. Many individuals also neglect proper warm-up and cool-down routines, increasing the risk of injury. Additionally, skipping strength training in favor of only cardio can create muscle imbalances, hindering progress. Others may fail to listen to their bodies, leading to overtraining. It's crucial to incorporate a balanced approach, remain consistent, and allow adequate rest for sustainable fitness improvements. Educating oneself on proper techniques and creating a well-structured plan can help mitigate these issues.