Thinking of you: the only occasion you set yourself
Every other bouquet has a date attached. This one has nothing but your decision to send it today, which is exactly where its strength comes from. Flowers that were not expected are remembered longer than flowers that were.
Usually it is distance
These orders are nearly always long-distance: someone is in another city or another country, meeting soon is not on the table, and the flowers stand in for a visit.
Which makes the time zone the first practical matter. A delivery booked for nine in the morning your time may reach the recipient at six a.m. or close to midnight. The slot is set in their local time, and it is worth checking that on the order rather than assuming.
Medium beats maximum
A very large bouquet arriving for no reason produces the question "what's happened?", and that question undoes the mood it was sent to create.
Fifteen alstroemeria, seven peonies, an armful of gerberas, a single-variety bunch of chamomile — that is the right scale. Save the expensive one for a date in the calendar.
Pick a weekday
Monday and Tuesday outperform Friday. Weekends are when everybody sends flowers; almost nobody sends them at the start of a week, and a bouquet on a Wednesday desk stands out more than any Saturday delivery.
The card carries this one
Here the words matter more than the flowers. "Thinking of you" on its own is a greeting card line; a specific detail is not. Naming the thing that reminded you of them today does more than any amount of wording, and that card gets read more than once.
Sign it. An unsigned bouquet with no occasion attached is a riddle nobody asked to solve.
What not to send
Red roses shift "thinking of you" into a declaration. Sometimes that is the intention; usually it is not. A calm palette says the thing you actually meant.
Frequency
One of these a quarter lands. One a week becomes wallpaper. If you send flowers to someone in Miami regularly, change the composition each time — a repeated bouquet stops being noticed faster than you would expect.