This year (5772) is the fourth year of the Shemitta cycle, and as such, this Pesach is the time of Biur Maaserot. Does anyone have suggestions (from experience or sources) regarding how to best dispose of one's Maaser Sheni coin?
According to this site there are 2 ways to take care of Biyur Maaser Sheni coins.
Destroy the coin to the point where it is no longer legal tender. You could bend it out of shape, shave off the face or cut it in half. Rav Dovid Lau suggests to beat it with a hammer until it's not recognizable.
According to the Chazon Ish you can redeem the value of the coin onto 2 spoons of sugar and then dispose of the sugar. Simply dissolve it - pour it down the drain with some water. This is also what Rav Avraham Yosef suggests.
Destroying the coin is also mentioned here.
I'm going to the Dead Sea tomorrow. Redeem your coin onto my coin (by adding your name to the comments of my blog http://rabbisedley.blogspot.com/2012/04/biur-maaser.html) and I'll be your shaliach for destroying maaser sheni.
This specific offer has expired, but the general approach of appointing an agent as sh'liach still applies.