1. Create an annual calendar. Be sure to include birthdays, anniversaries and other noteworthy dates for your clients.  Schedule alarms as reminders for 2 weeks in advance and send a card, email or gift as you feel appropriate.  Be sure to remember your clients during the holidays as well. 
2. Support your clients' favorite charity.  You can send a monetary donation in their honor, sponsor or host an event, donate an item for an auction, or simply attend their fundraising event as a show of support.
3. Offer to host a networking event for all of your clients.  Invite your clients and encourage them to bring some friends or colleagues and plenty of business cards.  They will appreciate the opportunity you are providing for them to gain more business.  Stand up and make a toast to the clients who attend, letting them know how grateful you are to know them and to work with them.  You may also get some more business for yourself from the people they invite.
4. Personalize gifts and tailor them to your clients' lifestyle.  For example, a person who loves the outdoors may appreciate a picnic basket, blanket and a bottle of wine.  A sports enthusiast may love tickets to their favorite team's next hometown game and a music lover may appreciate tickets to  a concert or to their local symphony.
5. Select gifts that are functional (and promotional).  There is nothing wrong with promoting your business on something you give as a gift, but make sure it is not just something that will be tossed in a drawer.  Consider giving a bottle of wine with your company logo and a personal greeting such as "Best of luck in your new home".  Other ideas may include a reusable shopping bag or a Post-It notepad holder for their desk.  As an example, my clients receive a custom printed Virtual Assistant "To-Do" list along with my email address and a note at the top that says "When you have 5 things listed, send your list to me".  Not only is this a cute idea, but it's a useful tool for my clients.  
6. Consider a customized gift.  Create a gift basket containing items for your clients' favorite team or sport.  For a coffee lover, you can give a basket with coffees from around the world along with an attractive ceramic mug for their office and a travel mug for their car.  You can also give a live plant to brighten their home or office, or if your clients don't live near you, send something that is specially known in your local area. For example, if you live in Florida and your client lives in New York, send a gift box with grapefruit and oranges.  
7. Offer  free time or credit in exchange for  their referral.  Who doesn't love receiving service for free?  Offer appreciation to your clients and their referral by rewarding them for their efforts.  My clients know they will receive either some work done without charge or a credit applied to their next invoice (whichever they prefer) when someone they refer to me decides they want to work with me.   And guess what?  Nearly every one of my clients has brought new business to me.  I know they are referring me simply for the service and support they receive from me and not for the reward, but I am pleased to thank them in this way.