To increment the security of the ASP.NET ViewState data you must enable the encryption of the data.

To do that, you can choose between two option:

1 - By the web.config file:
viewStateEncryptionMode="Always" />

Through this option you will enable the encryption of the ViewState data for all the application pages.

2 -  By the page:

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" ViewStateEncryptionMode="Always" %>

Make attention on the ViewStateEncryptionMode.

In this case you enable the encryption only for the page ViewState data.