I always forget the exact syntax and waste time searching for an example on the web.
   1:  $.ajax({
  2: type: "POST",
3: url: "Default.aspx/WebMethodWithParameters",
4: data: { param1: $('#val1').val(),
5: param2: $('#val2').val(),
6: param3: $('#val3').val()
   7:          },
  8: success: function(msg) {
9: //on success callback
  10:      },
  11: error: function(msg){
12: //on error callback
  13:      }
    14:  });