AngularJS Example:

HTML

<link rel="stylesheet" href="http://twitter.github.com/bootstrap/assets/css/bootstrap.css">
<div ng-app>
     <h2>Clearing ng-model</h2>

    <div ng-controller="EmailCtrl">
        <form name="emailForm" ng-submit="addEmail()">
            <input type="email" name="email" ng-model="userEmail" placeholder="[email protected]"> <span ng-show="emailForm.email.$invalid && emailForm.email.$dirty">invalid email</span>
<span ng-show="emailForm.$invalid">form invalid!</span>

        </form>
        <br/>
        <button ng-click="clearViaUndefined()">clear via undefined</button>
        <button ng-click="clearViaNull()">clear via null</button>
        <button ng-click="clearViaEmptyString()">clear via empty string</button>
    </div>
</div>

CSS

h2 {
    margin-bottom: 2em;
}
span {
    display: block;
    color: red;
}

JavaScript

function EmailCtrl($scope) {

    $scope.clearViaUndefined = function () {
        $scope.userEmail = undefined;
    };

    $scope.clearViaNull = function () {
        $scope.userEmail = null;
    };

    $scope.clearViaEmptyString = function () {
        $scope.userEmail = "";
    };
}