This is a really fun to do with your kids as you tuck them in for bed. When I was a kid I used to say, "Do the Treasure Hunt! Going on a Treasure Hunt! Please! Pleeease!" It's especially fun for ticklish people. It's called "Going On A Treasure Hunt"

[edit] Steps

First, have the recipient of the back tickle lay down face first as if they are going to receive a back rub.
This works best on bare skin (tickle factor) but either way is cool.
Say out loud, "Going on a treasure hunt..."
- simultaneously with your finger spell out "going... on... a..." by tracing big letters (one on top of another) on their back.
- Draw a big treasure box. (as you say "treasure hunt")
Say "X marks the spot"
- simultaneously draw a big "X" and "dot" the spot right in the middle by lightly stabbing it with your finger.
Say "Three miles down and..."
- Draw a big three, slowly draw a line down and a big arrow.
Say "... a question mark!"
- Draw a BIG question mark and dot it. (you could make a popping noise with your mouth as you dot it if you like sound effects.)
Say "A pinch..."
- Pinch 'em on the side (lightly! this isn't supposed to be painful)
Say "A squeeze..."
- Squeeze 'em a little. (Shoulders or side, whichever works best.)
Say "A tropical breeeeze..."
- Blow on neck. Optional, give 'em a little kiss on the neck first and then they can really feel the breeze.
Quickly tickle them all over their back!
By now they are surely giggling, so tuck 'em in, give'em a kiss and say "G'night"

[edit] Tips

Don't push hard, use quick light movements, like you are tickling with a feather.
Use an ominous voice or maybe a pirates voice for effect. Throwing all sorts of sound effects like wind, ocean, tropical birds or whatever you want is fun too.
Change it up the way you want it. One variation is just spelling things out and seeing if they can tell what you are spelling.
Great memories with this one.

[edit] Warnings

Only do the treasure hunt when they are down and ready to stay in bed.
Never ever do the treasure hunt tickle during the day "by request", save it for a reward at bedtime.
Try not to "do it again! do it again!" One time should suffice.