If you're throwing a large party and are concerned the punch will lose its sparkle over the course of a long evening, here is what I recommend. First of all, follow my tips above (these include not adding ice directly to the punch, keeping the ingredients very cold, and making the punch right before serving). Next, rather than using a double batch in a punch bowl, mix up a single batch in a pitcher, and keep ingredients on hand for a second batch. When the first pitcher is gone, just mix up a second batch. Because there are only a few ingredients, this should only take a minute or two. This way, the carbonation will stay fresher than if you made a double batch at the beginning of the evening.