<div class="portlet light bordered">
        <div class="portlet-title">
            <div class="caption">
                <span class="h4 caption-subject font-blue-steel bold uppercase">Preview</span>
            </div>
        </div>
        <div class="portlet-body">
            <iframe id="reportViewerFrame" style="width: 100%; margin: 0px;" frameborder="0" scrolling="no" onload="autoResize('reportViewerFrame')"></iframe>
        </div>
    </div>

$('.preview-button').on('click', function () {
        var labelID = $('#SelectedFederalLabel').val();
        var labelCount = $('#LabelsToPrint').val();
        var leftMargin = $('#LeftMargin').val();
        var topMargin = $('#TopMargin').val();
        var firstLabelPosition = $('#startOnLabelSliderValue').text();
        var vehicleID = $('#VehicleID').val();

        var url = "../../Reports/LabelViewerHost.aspx?mode=preview" +
            "&vehicleID=" + vehicleID +
            "&labelID=" + labelID +
            "&firstLabelPosition=" + firstLabelPosition +
            "&labelCount=" + labelCount +
            "&leftMargin=" + leftMargin +
            "&topMargin=" + topMargin;

        var reportViewerFrame = document.getElementById('reportViewerFrame');

        if (reportViewerFrame.src) {
            reportViewerFrame.src = url;
        } else if (reportViewerFrame.contentWindow !== null && reportViewerFrame.contentWindow.location !== null) {
            reportViewerFrame.contentWindow.location = url;
        } else {
            reportViewerFrame.setAttribute('src', url);
        }

        $(".preview-section").slideDown();
    });

protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack)
    {
        return;
    }

    _queryParameters =
        HttpUtility.ParseQueryString(
            HttpContext.Current.Request.QueryString.ToString());

    string mode = _queryParameters.Get("mode");

    ReportGenerationMode reportGenerationMode =
        (string.IsNullOrEmpty(mode) || mode == "preview") ?
        ReportGenerationMode.Preview :
        ReportGenerationMode.Print;

    ...
    Parse Query Parameters
    ...

    try
    {
        Microsoft.Reporting.WebForms.ReportViewer reportViewer =
            reportGenerationMode == ReportGenerationMode.Preview ?
            PreviewReportViewer :
            new Microsoft.Reporting.WebForms.ReportViewer();

        reportViewer.Reset();
        reportViewer.ProcessingMode = Microsoft.Reporting.WebForms.ProcessingMode.Local;
        reportViewer.LocalReport.ReportPath = Server.MapPath(_federalLabel.ReportFile);

        #region Set report parameters

        reportViewer.LocalReport.SetParameters(new Microsoft.Reporting.WebForms.ReportParameter("LabelsToPrint", _labelCount.ToString()));
        ... // I do this more

        #endregion Set report parameters

        reportViewer.LocalReport.Refresh();
    }
    catch (Exception ex)
    {
        // TODO
    }
}