> A tip from the boards: when taking a piss, take it over the top. ;)

Or alternatively, try applying an accent that isn't yours or theirs and see if they swallow it. This can provide hours of fun. Tell stories about the wildlife in South Africa or the vineyards of France using the necessary inflection.
Sometimes the satisfaction involved in pulling someone's leg doesn't involve the other understanding the hoax, but rather the look of bewilderment on the other's face :-)