I do it this way.  Another neat thing is the same EncryptBytes and
DecryptBytes methods will work with 3Des and Rijndael unchanged.  Just pass
the respective ICryptoTransform you need.

// Use the methods Sample:
           TripleDESCryptoServiceProvider td = new
TripleDESCryptoServiceProvider();
           // Set td key and iv here.
           string clearString = "This string will be encrypted.";
           byte[] clearData = Encoding.UTF8.GetBytes(clearString);
           byte[] encData = Utils.EncryptBytes(td.CreateEncryptor(),
clearData);
           clearData = Utils.DecryptBytes(td.CreateDecryptor(), encData);
           clearString = Encoding.UTF8.GetString(clearData);
           Console.WriteLine(clearString);

// Put these static methods in a Utils class or something.
 public static byte[] EncryptBytes(ICryptoTransform encryptor, byte[] data)
 {
             if ( encryptor == null )
   throw new ArgumentNullException("encryptor");
  if ( data == null )
   throw new ArgumentNullException("data");

  using(MemoryStream msEncrypt = new MemoryStream())
  using(CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor,
CryptoStreamMode.Write))
  {
   csEncrypt.Write(data, 0, data.Length);
   csEncrypt.FlushFinalBlock();
   byte[] encrypted = msEncrypt.ToArray();
   return encrypted;
  }
 }

 public static byte[] DecryptBytes(ICryptoTransform decryptor, byte[]
encrypted)
 {
  if ( decryptor == null )
   throw new ArgumentNullException("decryptor");
  if ( encrypted == null )
   throw new ArgumentNullException("encrypted");

  using(MemoryStream msDecrypt = new MemoryStream(encrypted))
  using(CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor,
CryptoStreamMode.Read))
  {
   byte[] fromEncrypt = new byte[encrypted.Length];
   int read = csDecrypt.Read(fromEncrypt, 0, fromEncrypt.Length);
   if ( read < fromEncrypt.Length )
   {
    byte[] clearBytes = new byte[read];
    Buffer.BlockCopy(fromEncrypt, 0, clearBytes, 0, read);
    return clearBytes;
   }
   return fromEncrypt;
  }
 }