I've actually profitably made heavy use of the phrase "friendly reminder" in email and text. As in:
Friendly reminder: your appointment with me is at 4pm this afternoon.
Just a friendly reminder, your appointment with me is at 4pm this afternoon.
The other thing you can do, which I also do, is to skip the preamble about how they might have forgotten, and simply reiterate the information they might have forgotten -- or otherwise slip it into the conversation:
I look forward to seeing you at our 4pm appointment later today.
A related trick is to include a reminder, clarification or request about something else entirely which happens to include the stealth reminder for what you're really concerned about:
I wanted to let you know, the ceiling projector is out of order in the room in which we're meeting today at 4pm. I thought I should mention in case you were planning on using it.
Hey, would you happen to have some spare dry erase markers you could bring to our 4pm meeting today? The conference room was out when I was there earlier.
Another trick, one which I don't use personally, but have seen others use, is to pretend one might have forgotten, oneself, and ask the party you suspect of forgetting for confirmation:
I'm sorry to bother you, but I think I may have written our appointment time down wrong. When did we agree to meet?
Yet another approach I use with my sometimes flaky boss, regarding our 1-on-1 meetings, is to ask, as if no commitment had been made, whether something is true (typically via SMS which is the only way to get him when he's in other meetings):
Are we meeting at 4pm today?
The nice thing about this approach is it lets him save face if he forgot and made other plans: he can just write back "No, not this week" or "No, I have to debargle the overthrusters" or "No, let's meet next week". He doesn't have to admit he forgot. Also, it doesn't make me look like I'm nagging him, the way sending reminders might.