Simple upload Form in Index View:

<form method="post" enctype="multipart/form-data">
    <input type="file" id="postedFile" name="postedFile" />
    <input type="submit" value="send"  />
</form>

In our Controller we now need a POST action like this: 

[HttpPost]
public ActionResult Index(HttpPostedFileBase postedFile)
{
    // FTP Server URL
    string ftp = "ftp://ftp.YourServer.com/";

    // FTP Folder name. Leave blank if you want to upload to root folder
    // (really blank, not "/" !)
    string ftpFolder = "test/";
    byte[] fileBytes = null;
    string ftpUserName = "YourUserName";
    string ftpPassword = "YourPassword";

    // read the File and convert it to Byte array.
    string fileName = Path.GetFileName(postedFile.FileName);
    using (BinaryReader br = new BinaryReader(postedFile.InputStream))
    {
        fileBytes = br.ReadBytes(postedFile.ContentLength);
    }

    try
    {
        // create FTP Request
        FtpWebRequest request = (FtpWebRequest)WebRequest.Create(ftp + ftpFolder + fileName);
        request.Method = WebRequestMethods.Ftp.UploadFile;

        // enter FTP Server credentials
        request.Credentials = new NetworkCredential(ftpUserName, ftpPassword);
        request.ContentLength = fileBytes.Length;
        request.UsePassive = true;
        request.UseBinary = true;   // or FALSE for ASCII files
        request.ServicePoint.ConnectionLimit = fileBytes.Length;
        request.EnableSsl = false;

        using (Stream requestStream = request.GetRequestStream())
        {
            requestStream.Write(fileBytes, 0, fileBytes.Length);
            requestStream.Close();
        }
        FtpWebResponse response = (FtpWebResponse)request.GetResponse();
        response.Close();
    }
    catch (WebException ex)
    {
        throw new Exception((ex.Response as FtpWebResponse).StatusDescription);
    }
    return View();
}

For uploading large files you can add this line to your web.config under system.web section (the default is 4 MB size limit).

<httpRuntime maxRequestLength="value in KB (f.e: 51200 = 50MB)" relaxedUrlToFileSystemMapping="true" />