First, you have to avoid getting distracted, because sometimes in a lengthy piece there's a tendency to daydream, get miffed at people coughing, or whatever. (I hear that some people, egad, even doze off.) It's important to be well rested, have no alcohol and only moderate carbohydrates before the show, not be hungry or thirsty, and be dressed in light comfortable clothing. It's also best not to plan any events for afterwards that might absorb your attention.
You also can't let yourself get distracted by the surtitles: it's important to read them in little bursts and sometimes just ignore them altogether. It helps me to sit in the balcony so I don't have to look up, away from the stage, at the surtitles. An ideal situation is to attend an opera you know well enough that you don't need the surtitles. I particularly like attending the encore performance of the Met HD program because the production is clear in mind from the first viewing.
Likewise, it's easy to focus too much on the singers and the visuals, and not pay adequate attention to the orchestra - occasionally I forget to listen to the orchestra, and I always get mad at myself when I do that. In some operas, like the Ring Cycle, I partciularly focus on the orchestra (it's not really possible to miss the singing or the action, in any event).
It's also important to me that I don't focus on the mistakes, but absorb it as a whole, uncritically. I can think critically later. This might separate me from people who are musically trained or more knowledgable about the music.
How much you know about the opera is a less definitive area. Opera can be enjoyed on many levels. For me, the decades-long process of understanding opera has been a gradual increase in awareness that deepens my understanding, and lately I've been speeding up that process, attending lectures and reading web sites and books. I'm getting more out of opera now, but I don't know that the emotional punch was any less when I was less knowledgable. I was lucky that my mother started me off on very accessible operas. My first was Maureen Forrester in Menotti's The Medium, which is not only very short but also really a play with words - and it was engrossing and spine-tingling. As I learned more my understanding deepened and my experience became richer. It took me until my 30s or 40s to like Wagner, although I was an early convert to Janacek.
For me, both the requirement and the reward is the high level of concentration required by opera, but there's more, and that's not overthinking the experience. The whole point of opera is the emotional impact, so you have to make yourself open to that. My dad once said that a game of golf is like living a lifetime, with emotional highs and lows - I think opera is for me what golf is for him. To a certain extent, you have to free yourself to ride the emotional rollercoaster, unhinge from the world and yourself. You have to be able to be caught up in the moment.
The difference between opera and a play or a book is that the emotional impact is not mostly the story. When I cry in an opera, it's almost always because of sublime music, not the plot. There are exceptions, such as the end of Gotterdammerung, when the final lietmotif heralding the era of man is almost overwhelmingly emotional on all levels. I've never been brought to tears by sublime music at the symphony - it's something about voice that is so powerful. And for me, the music has to be live... I'm not very enthusiastic about recordings, and can only watch videos in certain circumstances (Met HD productions I've seen before, and mostly, the Chereau-Boulez Ring Cycle).
All this is sort of dancing around my central interest in writing about opera: why does it affect me the way it does? That's a developing topic. I want to understand it in a way that will heighten the experience, rather than lessen it by externalizing a deeply felt experience.
I met someone recently who attends upwards of 100 operas a year, and it dawned on me that there is an addictive high to the experience, in much the way that joggers say they get addicted to an endorphin rush from extreme exercise. I guess I'm not an addictive personality... I'm happy with two or three dozen a year...