One of my very favorite things about the Reminders app is how you can ask it to prompt you about stuff in all sorts of situations. You can have it remind you at a certain time or when you arrive at or leave a place. But a relatively new feature that Reminders has is that you can have it poke you when you're messaging someone, so if you need to tell one of your friends about something but only want to do so when you're having a text conversation, you can have the app prompt you! How neat.
The way you create this on your iPhone, then, is to open the Reminders app and tap "New Reminder" to, well, make a new reminder. Once you get the title typed in, you'll see a little "i" appear next to it:
Tap that icon, and then you'll see the conditions you can set for your reminder. Here's where you'd configure the options I mentioned before—being reminded at a place, on a date, and so on—but the choice we're discussing here is labeled "Remind me when messaging."
Turn that switch on, and then you'll see the option to "Choose person"; touching that will open your contacts so that you can pick the one you want to be reminded about.
And that's it! When you're finished, tap "Done," and the next time you send a message to the person you chose, a reminder will appear to let you know that Marjorie needs to pick up some peaches. Mmmmm, peaches. If I knew someone named Marjorie, I would definitely ask her to do that, because that sounds amazing right now.