The more practice and experience you have under your belt, the greater your chance of capturing the moment. One useful piece of equipment I carry in my pocket is a micro cassette recorder. If the recorder has a voice activation feature, it will start and stop as you speak. Later, when you're at the light table and reviewing your slides, you can refer back to your choices and identify what works and what doesn't, thus eliminating making the same mistakes over and over. Record keeping will also help you get a feel for how your film reacts in different lighting conditions.
Film can record only a limited brightness range.
What looks good to your eye may not look the same on film. As you start to photograph in many different situations, you'll begin to identify which conditions record well. These are typically scenes with a five-stop brightness range when using color film. With a spot meter, you can be very precise in reading the difference between highlights and shadows. If the range exceeds the capability of your film, you have to choose where you want to retain detail.