There are several secrets to the little notes that can make such a difference to others.
The first is not to procrastinate. If you wait for more time or the right mood you may discover that the proper moment has been lost and the impact of the note has been diminished. If possible, write it shortly after you think of writing it.
Don't worry about obtaining nice stationery. Scrawl it on the back of a cocktail napkin, if need be. Delay is your enemy.
The note should have one purpose and convey a single feeling. Stir in another subject and you'll dilute the drink.
Strike the right tone. If you have doubts about a witty line, don't use it. It is better to be boring than dumb.
Make the note about the other person, not about yourself.
Mail it promptly. Speed will add to its impact.
Once you have mailed the note, forget about it. If you never get a response, that is fine. Just know that you have put something nice into someone's life. That should be more than sufficient.