Be clear about what you don't do. You may be willing to take anything, or you may only want to work on certain projects. Decide that for yourself and then make sure you know how to convey that to others. If, like me, you couldn't write advertising slogans to save your life because you know that it only brings out the cheesy side of you, be aware of that and avoid those opportunities. Meet your deadlines.
People often hire freelancers because they have emergency overflow work.
If you can make that first deadline, you'll not only have a happy customer but more work and an established reputation. Remember that a client's emergency becomes your emergency only if you agree to it. Providing extraordinary service in a client's crisis can be a great way to build your reputation. However, if a client is asking for unreasonable changes in the scope of work or the due date, you need to consider renegotiating what you get out of the deal. Give a polite but firm No when you need to.