This has probably been asked before here - but how does a developer leave a day job to start working freelance full time?

I've worked on projects in my own time on and off during the short 4 or 5 years of my career so far and I've always considered doing it, but how do people make that jump? How do you start working for yourself and how do you secure clients?

It takes time you don't just jump from full time employee to freelancer. You need to take it slow, start with one client and I don't think you need to secure clients if you give good service at reasonable price, in fact they will bring more clients to you if they are satisfied with your product/service.

If you are working full time, start with one client, build cliental base before leaving your job. It will take you minimum 3(can be less or more depends how you do it) months to build sufficient cliental base before leaving your job.