Everybody knows by now that you can use email autoresponders to send a birthday greeting to your customers (here’s a tutorial on sending belated birthday autoresponders). But why not pick a big-ticket item from your store, and send a triggered "happy anniversary" email? While you’re at it, throw in some useful tips for the customer, some feedback/social links, and upsell opportunities (in that order).
Click the thumbnail to zoom in on an example I put together in a few minutes.
No need to do this for every item in your store (that’d get annoying fast). Just pick one or two big ones, setup the autoresponder, then just put it on autopilot.