Think about how you might wish to use the work in the future in your teaching and scholarship. Think also about how your institution, colleagues and students might benefit from use of your scholarship on web pages and in a repository.
Thoroughly read the publishing contract. Be sure that you understand what you are giving away and what you are keeping. The future of your scholarly work is at stake. Get help if you need it.
Complete the author amendment, print a copy, and attach it to your publishing agreement. Write a cover letter to the publisher noting the presence of the amendment. Mail the contract, amendment and letter to the publisher.
Negotiate to keep those rights which are important to you. If a publisher is unwilling to accept the amendment, find out specifically why. Modify the agreement as you need to. If a publisher is unwilling to negotiate at all, weigh your priorities and decide whether it is more important to retain your future rights or place your work with this publisher.
Be sure to get signed acceptance of your amendment confirmed by the publisher. Keep a copy of all paperwork for your records.
Save the post print manuscript of your work. Even if you did not retain the right to post it to your web pages or institutional repository now, the publisher may grant those rights in the future.