There's some great, simple advice from the Freelance Folder in Seven Tips to Keep Your Clients Coming Back for More.  The tips:

1. Offer packages for recurring work.

2. Give your best clients special treatment.

3. Revive “zombie clients.”

4. Mark important dates.

5. Foster a feeling of belonging in an exclusive club.

6. Create promos throughout the year.

7. Ask for referrals.

Go read the entire article.  It is worth your five minutes.