I've been collecting since 1973, when I was seven years old. Back when I was a kid, I collected anything I could get, but of course I've had to narrow my scope somewhat! I've assembled a decent collection of mint U.S. stamps over the years, but since 2005, when I stopped trying to keep up with all the new U.S. issues, most of my collecting has been topical, reflecting such interests as J.R.R. Tolkien, runes and runestones, and various pop culture icons.
My main focus for the past twenty years has been stamps, stationery, and postal history pertaining to constructed international auxiliary languages. The lion's share of the collection deals with Esperanto, far and away the most successful language of this sort, but I've also managed to find postcards and covers featuring Bolak, Esperantida, Ido, Interlingua, Panroman, and Volapük.
Here's my most treasured piece from that collection (though not the most expensive): an 1888 postcard sent by Charles E. Sprague, the highest ranking Volapükist in the United States at the time, to the main office in Konstanz, Germany.