Keystone clients are important because they provide a skeleton for your business - a support system on which you can grow. They allow you to explore new creative avenues and even take on small, less lucrative or more ephemeral projects when they come along because, if something falls through, you know you'll have a reliable base of work to fall back on.
It takes a while to nurture these types of relationships but believe me, it's worth it.
Remember, every relationship starts the same way, at baseline, with a meeting. Where you take it from there is up to you.
Sometimes, keystone clients come from the most unlikely places. So, especially in the beginning of your freelance career, take people up on invitations to attend events. Get to know other professionals who show a keen interest in getting to know you. Nurture new relationships that arise from other colleagues' introductions. Even if what the potential client does is a bit "out of the box" for you, it never hurts to continue the conversation for a while. That experience might result in another introduction or to a new, exciting project.
I'm not suggesting you should exhaust valuable time and resources attending as many networking events as possible or that you continue conversations or negotiations with potential clients you feel instinctively incompatible with. I am suggesting an open-minded approach to networking. You never know where that random lunch with an old supervisor, touch-base phone call with an old friend or cocktail party invite might lead.
Practice controlled optimism. Who knows, that all-important keystone client could be sitting right across the table.