You market to past clients to maintain "top of mind awareness" so they'll not only call you when they're in the market for an agent, but refer friends and family to you.
But...
you can't just keep writing and saying "Remember me!" They'd get sick of that pretty fast and quit reading your messages.
So what can you say? I personally prefer a newsletter format, because unlike a letter, which should carry just one main idea, it allows you to add a variety of tidbits and jump from one subject to another.
It also allows you to offer something for everyone.
That's good, because obviously your past clients are not all alike.
Here's a list that should get your creative juices flowing:

• Market news - average prices, compared to last quarter, last year - or for a bit of human interest - 20 years ago.
  
• New developments - a new business coming in, new management at an old business, new housing developments, new recreational opportunities
• Retirement - News about someone well known in the community who is retiring, and perhaps an introduction to the person who will take his or her place
• Community events - Especially if you take part in one of your town's annual events.
  For instance: "Come see our float in the parade!"
• A calendar of upcoming community events
• A bit of personal news - Milestone events in your life, such as marriage, birth of a child, kids getting married, etc.
  
• Personal professional news - You attended a real estate conference, took an optional course, earned a designation, etc.
  - but do tell what that means to your customers if you mention it.
  
• Humor - everyone likes to laugh, just be sure to keep it tasteful!
• Quotes - My customers have actually called me to tell me a quote I used really hit home for them.
  Make sure what you choose reflects your own philosophy.
  
• Puzzles - if you consistently run out of ideas and need to fill some space, get a crossword puzzle creator.
  You'll have as much fun creating your clues as your readers will have figuring them out.
  
• Household tips - this one is a bit overused, but if you've got something good to share, do it.
  
• Personal observations - I once did a column that listed 10 or 15 things I love most about autumn.
  

And of course - offer a call to action.
Tell them how they (and their friends) can get a copy of a new report you've written, remind them that you're happy to do a free market analysis, etc.
This format is the "soft-sell," which I believe is far more effective in maintaining relationships than shouting "Use me!" every time you communicate with someone.
Probably 90% or more of your readers won't be in the market for your services on the day they read your newsletter.
So if all you're saying is "If you need a real estate agent...
" they'll toss your letter.
One more important element: your photo.
People forget names - and if you're just starting a newsletter program and haven't communicated with your past clients for a long time, they may have forgotten yours.
But if they see your smiling face, they'll not only remember you, but how they felt about working with you.
Since you're a top agent, that's sure to be a good feeling, so they'll read your message.
At that point they might even think something like: "Oh, Joe at work said he was thinking of listing his house.
I should give him this newsletter.
"