Hire on with a company.
Talk to everyone you can about wherever else the work, whatever else they do,
etc.
Remember and look up the companies when they have good things to say.
Look up the bad ones too.
Figure out how to apply to them. Sometimes you can ask whoever you are working with directly how to get a job at a company.
Figure out what makes a company bad and what makes them good.
While you're figuring stuff out, figure out how the "bad" companies are still doing business. (And let me know).
Remember names of people you work with and like. Remember those you don't. Learn how everyone is connected, (because everyone who lasts in this industry becomes connected).
Look up the people they mention, you never know when "My friend Tom" is Tom the guy in charge of hiring crew members for <large company>.
Probably look up everyone... Most people in
theatre are designers in smaller venues when they can -a lot of people have websites. See where else they've worked - it's not creepy to look it up, but talking to them about it might be.
Rinse and repeat.
Probably one of the best pieces of advice someone gave me; never say no to meeting someone and never turn down working with someone, once. You can always say no to the next one. (But don't let yourself be put into an unsafe situation.) Some of my best paying work has come from that one time a friend's friend was in need of a
hand at a cheap rate and I was the one that was available.