Immediate auto save of a form

Hi,

My client requirement is, if i change data in an lookupfield the must must save immediately automatically. I tried with 

function preventAutoSave(econtext) {

var eventArgs = econtext.getEventArgs();

if (eventArgs.getSaveMode() == 70) {

eventArgs.preventDefault();

}

}

but its nor working can any one help me out

Regards, and Thanks.