Most birthdays get acknowledged: a message in the morning, a card signed by people who barely know the person, a round of cake at 4pm. Few get genuinely celebrated — which requires noticing the difference between marking a date and making someone feel like the most important person in the room.
Make the gift impossible to regift
A gift that could plausibly be given to anyone says, on some level, "you were on my list." A gift that could only ever apply to this one person says something else entirely. At Galaxiana, naming a star and attaching a message specific to them turns the gift into something no one else could ever receive in quite the same way. From $34.99.
Bring up something they've forgotten
Most people underestimate how much they've affected the people around them. Tell them about a moment they probably don't remember — something they said or did that mattered to you at the time, and still does.
Let them choose, properly
Not "where do you want to eat" as a formality before going where you'd planned anyway — actually let them pick, even if it's not what you'd have chosen. A birthday is one of the few days that's allowed to be entirely about what one person wants.
The formula, if there is one, is simple: pay specific attention, and make the effort visible. Everything else is decoration.