Play.
Some people, I think, tend to do only rigid technique/application/form practice, or sparring, and not much in-between. But I think the in-between -- "playful sparring," or open-ended flow drills, where the pressure is low enough that one can experiment with new ideas, is where the learning takes place. If you're always practicing all or nothing in intensity, I think you tend to only develop a small and narrow focus and number of techniques and principles that you can apply, because you don't have that combination of a resisting partner and a low-stress environment where you can play and figure out how things do and don't work.
The other thing I would recommend is diversify your practice, and don't approach sparring with a competitive mindset. Approach it as an experiment, and be willing to try things that might not work. You learn more when things don't work -- just be careful not to judge the value of the technique or principle that you weren't able to apply in the moment. Rather, try to look at the whole picture -- there are a whole myriad of reasons why something works or doesn't work -- you may have been using it inappropriately, at the wrong height or on the wrong line or against the wrong kind of stimulus. Or, you may simply have an opponent who instinctively or otherwise knows how to counter or shut it down, and you need to be able to flow to the next position. Failure often shows us how and why something works, and by being careful about our analysis and withholding judgement, we can develop a more complete and solid understanding of how things work and when they're appropriate.
I also really encourage people to practice with people from other martial backgrounds. You'll learn a lot about yourself and your art by experiencing other martial arts, and touching hands with people who practice different systems, based on different principles -- or even, people with little or no experience in martial arts at all. They will do things differently that catch you off guard at first, and you will need to learn to adapt to them.
Lastly, and most importantly, keep track of the bigger picture -- the principles and concepts that your system aims to apply. People often overlook the most basic of principles in sparring, and begin to just act instinctively as opposed to purposefully learning to apply this or that concept or principle. Don't get so lost in the trees that you loose sight of the forest. When I evaluate myself from a Wing Chun perspective, I ask myself: Was I adhering to centerline theory? Was I chasing center, or chasing the hands? Did I have sufficient forward pressure to sense the gaps in my opponent's defenses, and did I stay close enough to him? Was I comfortable enough with my own skill to maintain a distance that suits my style, or did I keep backing up because I wasn't comfortable?
I really believe that you have to be willing to fail, and you should approach "sparring" as nothing more than an experiment, each and every time. Try out different things and stick to your hypothesis each time, and care less about your performance as a result. In order to really learn to apply something, you have to be willing to fail plenty, and you have to "stick to it" even as you fail. If you can approach it as nothing more than an experimentive learning process each time, and you take the time to form, isolate, and test your hypothesis, you'll learn a lot.
Make your development in sparring a conscious one, and not an unconscious one. Don't just "fight" or "play" instinctively. Carefully and consciously develop what instincts and what habits you form through your sparring, as opposed to letting them develop haphazardly.