*Disclaimer* - this is priveledged info for DM's eyes only - if you're not a DM and you read this, we will find you! ;-)
"Footsteps" is a little trick I picked up far too late in my experience as a DM. I still remember the first use of Footsteps: I was running an original adventure I had spent hours on, meticulously tooling and retooling every detail, every NPC, everything. And yet, for whatever reason, my players were having an off session. Their attention wandered, no notes were taken, the map was haphazard at best, and as they missed detail after detail, my own interest in the session began to waver.
Now, when I get bored, I begin to mess with people. Its just the way I am, and in this instance the players happened to be the most convenient to mess with. Picking out the most distracted player at the table, I hastily scribbled a note on a scrap of paper, and made a big production of passing it to him "secretly" (which of course heightened everyone's level of alertness in and of itself):
"You think you hear footsteps behind you, but when you turn to look, no one is there."
The player nodded sagely, tucked the note into his PHB, and handed a note back to me.
"I continue forward, but listen carefully to hear these footsteps again, ready to spin with weapon out if I do."
I read his note, nodded seriously at him, and continued on with the adventure. A few minutes later, I passed another note:
"You hear the footsteps again".
The player yelled out an alarm to the other players, and spun around with bare blade to see nothing. I passed him another note.
"As you spin around, you see nothing, but you feel a hot breath on the back of your neck, as if whatever is stalking you has spun around behind you."
This led to much searching around, the waste of a few spells, the checking of coin purses and backpacks, as the players frantically sought to determine what was hunting them. Of course, there was nothing to find, and eventually the adventure continued on - but I noticed the difference immediately - the players paid attention to everything! No detail was too small to escape their notice, everything was carefully noted and mapped, and the adventure proved to be a smashing success. So much so, I was surprised when it was over that no one even remembered those elusive Footsteps...
I took the lesson to heart, and began to introduce various versions of "Footsteps" in almost every adventure, regardless of the interest level. Some variations included:
"You notice faint, bleeding pinholes on your left arm."
"You suddenly feel light-headed."
"You feel as if invisible fingers have just brushed across your cheek."
Used wisely, and in a timely fashion, "Footsteps" is a great tool for any DM...