Web Forms :: Class Variables Which Initialize The First Time Execute Page Load?

I have some code I only want to execute the first time I display a form. With this in mind I created a class variable which I initialize the first time I execute page load.

public partial class CLVideoDefinition : System.Web.UI.Page
{
string videofile;
string first;
protected void Page_Load(object sender, EventArgs e)
{
if (first != "N")
{

I then select a value from a drop down list , but when it does the auto postback it executes page load but my class varaible has been reinitialized to null and will then execute the code I only want to execute on the first run. why my class variable has been reinitialized.