If you're not a manager already, you'll soon learn that making good hires is so, so hard. In the tech space, just finding qualified people is hard. Then you have the huge variable (for both parties) of how they'll actually function in your particular environment. Having access to a team you know can work with you and each other well is extremely important to your ability to succeed, particularly in a new project or venture.
Here are a few specific things I've focused on:
1. Identification & Focus
Keep your eyes open all the time. I did this even when I was in a very junior position. I remember as a sales engineer I was asking a developer for help at a trade show. He helped me fix the issue at and then when I was about to move on he kind of scolded me and said "Well if you really want to understand what happened there, you should really [xyz]". I did xyz and I remember thinking, "This guy is [my kind of] awesome. He cares about the work and he cares that other people care, too."
Now, I'm not saying that's the specific thing you should look for when you're trying to build your network. We all have our particular strengths and weaknesses and the people you're going to have a reason to collaborate with will vary on the kind of project you want to go after. But I would pay attention to those little details. Don't just focus on the person everyone says is a superstar. They'll be good at something, but that may not be the particular something that matters to you.
More frequently, I hear speakers, etc. putting forward alternatives to the idea of the 10x engineer, 10x marketer, etc. They use metaphors like a bank heist (Oceans 11, for example) or a movie production. You need all different kinds of people and you need to achieve that alchemy of them working well together. That's what's important.
What I don't particularly recommend is volume. Trade shows, meetups are great for their purpose. But just meeting a lot of people and connecting on LinkedIn isn't likely to be that valuable. Look for meaningful interactions that map to what your doing now, which brings me to my next item...
Find ways to work on projects with those kind of people above. It will be worth it even if it takes you a little off topic or involves extra work.
Make the extra effort to have a drink with those people or give them a call even after once of you goes to work somewhere else. If they're super busy, try to catch them on their drive home. Make the extra effort- it will be worth it.
The stronger your network is relative to its nodes (other than you), the better it is. Find out what's on your key network's A-list and make the extra effort to connect them with other people from your network.
That doesn't mean just randomly introducing them to each other because you think they're both great and wouldn't they just make the best of friends. That can be okay in a social setting where both parties have the opportunity to flee, but super awkward in an email. Instead, really make the effort to find out specifically what person A is working for and then introduce them to person B with a very specific thing they can discuss together.
SIDE NOTE: You know, one thing I hear but I would like to know more about is the difficulties women encounter in this and how they manage through it. Basically, what I hear is that a lot of the relationship building, particularly outside of work, can sometimes be awkward because (and yes this is lame/stupid) there is the possible perception of romantic involvement. Personally, I would be interested to hear from some women on the relevance of this and what they think managers and colleagues should do to help women build their professional network.