When doing web testing using Watin, it is not trivial to be able to do a POST request to the server. However, with the help this article on microsoft.com, I was able to figure out how. I ended up with writing this class:

public class Navigator
{
    private IE _ie;

    public Navigator(IE ie) { _ie = ie; }

    public void Post(Uri baseUri, params KeyValuePair<string, object>[] postData)
    {
        object flags = null;
        object targetFrame = null;
        object headers = "Content-Type: application/x-www-form-urlencoded" + Convert.ToChar(10) + Convert.ToChar(13);
        object postDataBytes = MakeByteStreamOf(postData);
        object resourceLocator = baseUri.ToString();
        IWebBrowser2 browser = (IWebBrowser2)_ie.InternetExplorer;
        browser.Navigate2(ref resourceLocator, ref flags, ref targetFrame, ref postDataBytes, ref headers);
        _ie.WaitForComplete();
    }

    private static byte[] MakeByteStreamOf(KeyValuePair<string, object>[] postData)
    {
        StringBuilder sb = new StringBuilder();
        if (postData.Length > 0)
        {
            foreach (KeyValuePair<string, object> postDataEntry in postData)
            {
                sb.Append(postDataEntry.Key).Append('=').Append(postDataEntry.Value).Append('&');
            }
            sb.Remove(sb.Length - 1, 1);
        }
        return ASCIIEncoding.ASCII.GetBytes(sb.ToString());
    }
}

For example, I can use it like so:

using (IE ie = new IE())

{

    Navigator navigator = new Navigator(ie);

    navigator.Post(new Uri("http://www.foo.com/"), new KeyValuePair<string, object>("p", 1));

    Assert.AreEqual("OK", ie.Text);

}