When you're a freelancer, over the years, you get to know other people in your field. I've been pretty lucky because I've been in the publishing business for a long time.
I work full time as a senior editor in the health care field. I've made lots of contacts with other editors, proofreaders, and writers who work full time and/or freelance. I've worked in many fields: legal, financial, engineering, education, computer, and medical.
My rolodex and e-mail address book are full of contacts from various areas of expertise. Whenever you can't take a job, offer to recommend someone else. Before you do, though, make sure that the person is reliable and right for that assignment. Have the freelancer you recommend let you know if that assignment worked out. Follow up with the client to find out if someone (doesn't have to be the contact you recommended) was chosen for the assignment. If not, offer to recommend someone else.
Contacts also come in handy when you're working on a project and are striving to finish it in time, but you hit a snag. Even though I haven't tried this personally, some freelancers hire someone else to work on a portion of the job. This is risky. You should be familiar with that person's work. Even if you are, you should review the work, because overall you're responsible for that project.
Other times if you're editing a book, for example, the client might ask you if you can do the index. Be honest if you don't have the expertise for indexing. But offer to recommend someone who will do a good job on it.
Besides getting contacts from your previous jobs, conferences and professional associations are also good sources. Associations like the Editorial Freelancers Association, Media Bistro, and LinkedIn are invaluable. So start networking and compile your contact list. It'll come in handy one day.