And while the courteous thing to do would be to send a letter, or even give them a phone call, I doubt that you - if you're not using an agent - have a legal responsibility to provide much more than "I'm sorry, I've decided to go with a different applicant".
You might check with a source such as Nolo.com, for a generic legal suggestion. They have great books for landlords - here's a link to a bundle you might find helpful.
Depending upon where you or your rental are located, you SHOULD consult a local Realtor, if for no other reason than to request a recent rental survey of the market for your property - rents have been going up pretty well, over the past few months.
Good luck in finding solutions to your situation.
You can talk with your agent about getting the denial form if it is for credit reasons. Its a new form that came out in April 2 months ago. Check with your agent about your situation... How else can I help you..
Ingrid Ski Realtor
Lynn911 Dallas Realtor & Consultant, Loan Officer, Credit Repair Advisor
The Michael Group - Dallas Business Journal Top Ranked Realtors
Best of luck,
Newport Beach, CA