There are multiple ways of doing this ie using JavaScript, but this is the server side solution.

You can adapt this method for several other solutions:

        foreach (Control c in this.Page.Controls)
        {                        
            if(c.GetType().FullName.Equals("System.Web.UI.HtmlControls.HtmlForm")) {
                foreach (Control cForm in c.Controls)
                {                        
                    if(cForm.GetType().Equals(typeof(TextBox))) {   
                        ((TextBox)(cForm)).Text = "";
                    }
                }
            }
        }