Make enquiries from literary agents and publishing houses as soon as you have agreed to write the biography and written a synopsis. Pitch your idea and send a proposal along with your ideas about the possible market for this book. Conduct research on potential readers, as publishers appreciate writers who have some commercial awareness and who are prepared to help promote their book.
Approach a literary agent who specializes in the non-fiction genre, particularly if she represents other biographers. A literary agent can be a useful ally in negotiating a good deal. Use the positive aspects of your past work and your proposed biography to make the best possible deal. If you have had past commercial success, current professional contacts, a website or even marketing experience, these can help your negotiations.
Familiarize yourself with the different types of deals that may be offered. Some publishers are moving away from an advance followed by a percentage of the sales. You may be offered a flat fee, or even a higher royalty, instead of an advance. Carefully weigh up each of these options. Ask the advice of a financial adviser or an accountant as there are tax implications for each of the options.