Most women wait for the perfect opening and miss the moment. A simple situational question is usually stronger than a rehearsed compliment because it feels natural and gives him an easy way to respond.
Ask about the event, the drink, the speaker or the place. Then listen for something you can follow up on. Conversation is less about delivering a perfect sentence and more about making the next sentence easy.
Keep the exit light
If the energy is not there, smile and move on. A relaxed exit protects your confidence and makes approaching the next interesting person much easier.