The short version of this writeup:
1) Address and subsequently apply postage to your mail
2) Put the now ready-to-be-mailed mail in a package - a box, an envelope, doesn't matter as long as it fits
3) Address the package to the post office in a remote city with an interesting name
4) Make sure you apply postage to the package, and also make sure you allow extra time to mail your articles (because first, they have to arrive)
5) Mail said package of letters; take it to the post office
6) Be patient, and watch your recipients' faces quirk as they receive mail with a postmark from a place that you're nowhere near
Or, the short short version:
1) Mail your preposted letters in a box to another post office, where they will open the package and send your mail.
While this seems like a fairly useless technique...well, OK, it is fairly useless. But it has its novelty.
As a for instance, you can send your St. Valentines' day card to your loved one up the road from you by way of Loveland, Colorado by this technique. Though if it's just one card, you probably don't need a box. This is how I learned of this particular postal hack - my mother (we're from a divorced family) would do this same thing with my siblings and myself.
But there are other interesting things you can do. Such as sending your Christmas cards through places like North Pole, Alaska, Christmas, Florida, Bethlehem, Pennsylvania, Noel, Missouri, and the like. By a long shot, Carpinteria, California might also work - only because of the Santa Claus Lane exit from US Highway 101. Any other time of the year (or, if you get it in early enough before they have to put the machines to work), they will hand cancel the articles at the station and send them on their merry way; Elsewise, they send it off to the nearest processing and distribution center to be machine cancelled with the appropriate city's cancellation mark.
How all this works is simple. First thing you do is get your mail ready to be mailed - address, seal, apply postage as necessary. You take said mail, and rather than dropping it off as is with your letter carrier, you pack it into a package of some sort (box, satchel, tyvek envelope, Art Garfunkel's luggage, etc.), apply postage as necessary to this package as well, address it to the appropriate post office (you can usually get away with just "United States Postal Service/(Name of special or hand cancellation unit)/City, State Zip Code", but it makes it easier to locate if you provide the street address as well). They pick it up from there - they'll hand cancel it and send it out. It might help to enclose a note on top of the mail explaining what you want done.
By extension, of course, there are other things you can do with this. Such as sending your tax returns to the Internal Revenue Service via Sydney, Australia. Or sending your wedding invitations to Truth or Consequences, New Mexico. St. Patricks Day cards? Send them through Dublin, California. Or play a very cruel joke on somebody whose loved one has recently died - send them a postcard from Hell, Michigan or Hell Hole, New Mexico. Or send a postcard any ol' time of the year to Fred Phelps through either of those two latter locations. ("See you here soon!")