One thing I keep forgetting to reorder is vitamins.
We get a specific one that comes in a 60-day supply. So when we notice it's running low, I add it to my todo list (the loyalty account is in my name).
Problem is, I don't get around to ordering right away or forget about it when I'm not working, which means we get close to running out now and then.
This product is ideal for a subscription but not every consumable product has the set reorder timing that subscriptions use. Also lots of people are adverse to subscriptions and they can take a lot of setup work for you to offer.
In those cases you can use a simple reminder.
Most email marketing systems let you setup emails to go out on a set schedule (e.g. 68 days after an order). Use that and write up a simple reminder email to nudge the customer to make a purchase.
If you don't know how long to wait for a replenishment email, calculate your Average Latency and use that (or have Repeat Customer Insights calculate it for you). It's a great starting point for your reminders and you can adjust the timing as you go.
(These emails are called replenishment emails if you want to lookup examples but don't get too busy looking for the perfect one. Something simple if better than nothing)
In my case, this email would help me remember "oh yeah, I got to reorder" and will be visible in my email until I do it. A good email could even have a link that copies my old order into the cart so I can easily checkout. Shopify used to have a built-in way to do that but I can never remember the name of that feature (it's buried in some help docs somewhere).
Using these emails can help your customers remember a purchase they might forget about. Done correctly, the customer might even thank you for the reminder.
It's always nice to be thanked when asking a customer to buy again.
Analyze your customer behavior
A quick and automatic way to measure your customer behavior is with Repeat Customer Insights. It'll automatically import your Shopify data and run a number of analyses to highlight different customer behavior.