WP7 HttpWebRequest without caching

It seems that HttpWebRequest caching in WP7 is enabled by default, how do I turn it off? Adding a random param url + "?param=" + RND.Next(10000) works, but it's quite tricky and I'm not sure if it will work with all servers.

For future reference , this worked for me ( I could not use additional query parameter due to project requirements) :

        HttpWebRequest request = HttpWebRequest.CreateHttp(url);
        if (request.Headers == null)
        {
            request.Headers = new WebHeaderCollection();
        }
        request.Headers[HttpRequestHeader.IfModifiedSince] = DateTime.UtcNow.ToString();

In case of HttpClient (Portable for Windows Phone) "Cache-Control": "no-cache" on server side works only sometimes. And I cannot add query string random value to RESTful api call as well.

Solution from @frno works good and looks like for HttpClient:

client.DefaultRequestHeaders.IfModifiedSince = DateTime.UtcNow;

Thank you.