These solutions are all very well if you only have one dialog in the page at any one time, however if you want to style multiple dialogs at once then try:
$("#element").dialog({
buttons: {
'Save': function() {},
'Cancel': function() {}
}
})
.dialog("widget")
.find(".ui-dialog-buttonpane button")
.eq(0).addClass("btnSave").end()
.eq(1).addClass("btnCancel").end();
Instead of globally selecting buttons, this gets the widget object and finds it's button pane, then individually styles each button. This saves lot's of pain when you have several dialogs on one page