Email. If you do one thing thats very important w/ your ecom store its to email your list.
Most important are your email sequences. As well as random "promotions" and depending on the product, even "you've won randomly been selected to win X, just pay shipping to receive it" types of emails work great.
Make sure you have email sequences setup and in place from the time someone buys or even abandons the cart to long after their purchase.
5 or 10 emails will help but going above and beyond and sending valuable content instead of just shoving sales down their throats will yield awesome results down the road when that email sequence is making you money with no pretty much no effort after you write them. Or hell, pay someone to write em.
So if that's you who doesn't email - stop being that lazy fucker and actually do it. I know way too many people who leave tons of money on the table considering how many clients they have by simply not setting up emails to cross sell and promote their other products.
Ok thx now go write some emails.