An excellent direction from @eu-ge-ne.

I have used the idea of custom route from @eu-ge-ne to add the route value to every url and used a basecontroller to handle the Preview key in session.

if ((requestContext.HttpContext.Request.QueryString != null &&
     requestContext.HttpContext.Request.QueryString["Preview"] != null &&
     requestContext.HttpContext.Request.QueryString["Preview"].ToString() =="True") ||
     (requestContext.HttpContext.Request.UrlReferrer != null &&
     requestContext.HttpContext.Request.UrlReferrer.ToString().Contains("Preview=True")))
    {
         //Add the preview key to session
    }
    else
    {
         //Remove the preview key to session
    }

I have used the above code in the Initialize method of the base controller. This way the preview key will in session if the querystring has Preview, else it removes from the session.

Thanks to @eu-ge-ne once again.

Maybe you need a custom route?:

public class PreviewRoute : System.Web.Routing.Route
{
    ...

    public override VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values)
    {
        var preview = System.Web.HttpContext.Current.Session["Preview"];

        if (!values.ContainsKey("Preview"))
            values.Add("Preview", preview);

        var path = base.GetVirtualPath(requestContext, values);

        return path;
    }
}

}

Set Session["Preview"] at any time and you will get all your urls with ?Preview=True:

System.Web.HttpContext.Current.Session.Add("Preview", true);

UPDATED:

Use this route in the Global.asax.cs:

routes.Add("Default",
    new PreviewRoute("{controller}/{action}/{id}", new MvcRouteHandler()) {
        Defaults = new RouteValueDictionary(
            new { controller = "Home", action = "Index", id = "" }
        )
    }
);

instead of:

routes.MapRouteLowercase(
    "Default",                                              // Route name
    "{controller}/{action}/{id}",                           // URL with parameters
    new { controller = "Home", action = "Index", id = "" }  // Parameter defaults
);

Also you can try this extension:

public static class CustomRouteExtensions
{
    public static void MapPreviewRoute(this RouteCollection routes, string name, string url, object defaults) {
        routes.MapPreviewRoute(name, url, defaults, null);
    }

    public static void MapPreviewRoute(this RouteCollection routes, string name, string url, object defaults, object constraints) {
        if (routes == null) {
            throw new ArgumentNullException("routes");
        }

        if (url == null) {
            throw new ArgumentNullException("url");
        }

        var route = new PreviewRoute(url, new MvcRouteHandler()) {
            Defaults = new RouteValueDictionary(defaults),
            Constraints = new RouteValueDictionary(constraints)
        };

        if (String.IsNullOrEmpty(name)) {
            routes.Add(route);
        }
        else {
            routes.Add(name, route);
        }
    }
}

In Global.asax.cs:

routes.MapPreviewRoute(
    "Default",                                              // Route name
    "{controller}/{action}/{id}",                           // URL with parameters
    new { controller = "Home", action = "Index", id = "" }  // Parameter defaults
);

You could create a view Helper that appends the existing query string onto any links you create with your new helper.

This may help

You might be better storing this information in session.