Common mistakes that can make a presentation ineffective include reading slides word-for-word, turning your back to the audience, talking too fast, fidgeting, not preparing well, ignoring the audience, using inappropriate or overly verbose content, including too much text on slides, lacking dynamism in delivery, and avoiding eye contact. Proper preparation, engaging delivery techniques like movement, concise yet informative visuals and content, and focusing on audience needs can help create an effective presentation.