This way you will remember from whom you got the name. This letter will differentiate you as being courteous and professional. Both of these perceptions improve the contact's level of trust and raise the probability of sending a referral. Included in this package should be a cover letter, a business card, a practice profile and a copy of a recent article you have had published. Even if you gave a business card at lunch, send another one.
Business cards taken at lunch often end up at the dry cleaners or in a dresser drawer.
The business card that comes with your letter will probably go in the rolodex. Your practice profile should be an inclusive document that outlines your name, firm, address, phone, fax, education and honors, the focus of your practice with brief descriptions, your professional leadership including organizations and positions held, lectures and publications. This is not an employment resume. Don't include any firms other than your current firm. You should also describe civic leadership including business or civic organization and positions, boards of directors, charity volunteerism, etc.