You should also include paper and font samples for your customer to select from, any stock graphics that you work with, plus the graphics you've created or the logo reproductions done from scans.  The more complete the options and examples, the easier it is to determine the exact requirements of your customers project.

Your visual demonstration through your portfolio will encourage your client to use your services.  The convenience of being able to see and handle the paper stock, to select the exact style of font and the selection of graphics available makes determining the entire project quick and easy, letting the customer make all of the decisions.  This will reduce any error of miscommunications or the inability on the clients part to adequately describe their needs.  All you really need to do then is note the text content, the selected paperstock, the font style requested, and the layout design they wish you to follow.

You may also want to keep any letters of recommendation, thank you cards and notes, or articles written about your services for your prospective client's inspection.  This is helpful when your word is not enough to appease an uneasy client, nothing is more powerful than the recommendation of past clients.