The following codes work for me. Define two fade-in and fade-out functions and put them in each other's callback.
var fIn = function() { $(this).fadeIn(300, fOut); };
var fOut = function() { $(this).fadeOut(300, fIn); };
$('#element').fadeOut(300, fIn);
The following controls the times of flashes:
var count = 3;
var fIn = function() { $(this).fadeIn(300, fOut); };
var fOut = function() { if (--count > 0) $(this).fadeOut(300, fIn); };
$('#element').fadeOut(300, fIn);