All good and practical answers, but if you want a different legal perspective, here is a good talk with a memorable title from an agency that has worked with many clients over the years (warning, there is some foul language):

https://www.youtube.com/watch?v=jVkLVRt6c1U

Mike Monteiro is the design director at Mule Design Studio. During a seminar at CreativeMornings, Mike gave a really insightful and thorough speech about how a creative professional can protect his or her work and payment in the event that something goes wrong with the client or job. This video is long but it should at least make you aware of issues you might face on your journey, including:

Have you ever had a client dismiss a project after you have already put in hours of work?

What are you supposed to do when a client brings in a second professional to also work on your project?

What can you do if a client says your work isn't really what they were looking for (and doesn't want to pay you) after you have already delivered the final project?

Mike's conclusions: Hire an experienced lawyer, get your contracts drafted right from the beginning, and use pressure to get what you are entitled to. Don't be a bottom.