A good conversation requires mutual interest.
Your interest:
1. Only approach women you REALLY want to meet. Don't force yourself to approach just for the sake of it. That makes it mechanical and women pick up on that. Be a sniper. Use your gut to figure out who you REALLY want to talk to. MOST IMPORTANTLY, let your gut instinct tell you who WILL LIKELY BE RECEPTIVE to your APPROACH. Random approaches where you roll the dice is ok and can work sometimes, but ideal is to approach women you have a good FEELING about.
Her interest:
1. This is the part that we don't control. Obviously, do what you can to optimize your look. Start the conversation and you will know in a few minutes (max) if they want to talk to you or not. Girls that find you attractive will HELP with the conversation. Girls that don't like you will give you one word answers and let the conversation die out. Don't be afraid to eject if conversation goes nowhere.
General conversational tips:
1. Have a hobby or interest you are passionate about. I play the ballroom dance card. Yours might be skiing, martial arts, guitar, rock climbing, traveling, writing, or whatever. Find a way to drop it in the conversation.
2. Observe your surroundings. Talk about the music or the venue or other cool places you've been to.