Regarding putting it in writing, once you and your client have made an agreement, don't forget to update that agreement if/when changes are made.

I had an agreement once when I was young and inexperienced where I was to be paid so much an hour, with a maximum amount to get a certain job done.

The client then came up with ideas to improve the product, and in the excitement of implementing these changes I failed to modify our agreement.

At the end of the project I had a lot more hours than the client had agreed to pay for and no way to force him to pay. An expensive lesson for me!