Foreach statement for two OfType
You could write it in 1 loop but you'll still need separation of logic for each type of Control:

foreach(Control c in panel1.Controls)
{
    if(c is TextBox)
    {
        var tb = c as TextBox;
        if(!String.IsNullOrEmpty(tb.Text))
            tb.Text = "";
    }

    if(c is Label)
    {
        var l = c as Label;
        if(l.Tag != null &amp;&amp; l.Tag.ToString() == "Clearable")
            l.Text = "";
    }
}