First step, don't call yourself a freelancer, cuz you don't work for free. The word has connotations of "temp". A freelancer is someone you bring in for a 3 week stint. A contractor or consultant is someone you bring in for a longer project. It doesn't matter that these are all basically the same person, but perception matters.
Always make the relationship a business to business one. Don't get stuck being the "hired hand". Just doing the job isn't enough, always provide more value, get involved in their biz if you can. Don't work from home, work on site, "out of sight out of mind" is a thing, and is the fastest way for a client to see you as expendable.
The best way to find work is to build a network. The best way to build a network is to get a gig with a bunch of other contractors at it. Then work hard, and be pleasant to work with. Once that gig dries up, and all those other contractors go out to find new gigs, if they liked you, they'll recommend you for openings at another client. Rinse and repeat.
I got very lucky in that my first contract was a 6 month contract on a team of 15 other contractors (at a F500 company). That 6 months turned into a year and when the money ran out, I had a network of 15 other people to lean on. Since then I've never had to look for a gig, they've all come to me.