Less clients = more money. I made most money from the clients that I was with for 40 hours a week inside their company than I could ever make from clients that wanted to hire me 20 hours each.

Also, your first instinct will be to charge less if they want you longer. But if you freelance not only for the money but also for the freedom, you might want to charge more if they want to hire you for a longer (multiple month) period.

Lastly, you will slowly die as a freelancer if you only do what you would do as an agency or as an employee. You have to make sure you add some additional value. Teach your clients something while you're there. Suggest things for their design culture. Do more than just the work.

Build your pipeline of future clients, both near and medium term. Even when you're busy, find time for coffees and phone calls with people you're not ready for. Be choosy about who you want in that pipeline, and those discussions will be more enjoyable. And don't be afraid to let people know when you've got time opening up; when someone you talked to last month knows you're available in a couple weeks, they could well be ready to have you step out of one gig and into another without breaking a sweat. That was a really hard one for me to learn when I was freelancing.