How to Persist Variable on Postback

I created a single page (with code behind .vb) and created Public intFileID As Integer

in the Page load I check for the querystring and assign it if available or set intFileID = 0.

Public intFileID As Integer = 0

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If Not Page.IsPostBack Then
        If Not Request.QueryString("fileid") Is Nothing Then
            intFileID = CInt(Request.QueryString("fileid"))
        End If

        If intFileID > 0 Then
            GetFile(intFileID)
        End If
    End If
End Sub

Private Sub GetFile()
    'uses intFileID to retrieve the specific record from database and set's the various textbox.text
End Sub

There is a click event for the Submit button that inserts or updates a record based on the value of the intFileID variable. I need to be able to persist that value on postback for it all to work.

The page simply inserts or updates a record in a SQL database. I'm not using a gridview,formview,detailsview, or any other rad type object which persists the key value by itself and I don't want to use any of them.

How can I persist the value set in intFileID without creating something in the HTML which could possibly be changed.

[EDIT] Changed Page_Load to use ViewState to persist the intFileID value

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If Not Page.IsPostBack Then
        If Not Request.QueryString("fileid") Is Nothing Then
            intFileID = CInt(Request.QueryString("fileid"))
        End If

        If intFileID > 0 Then
            GetFile(intFileID)
        End If

        ViewState("intFileID") = intFileID
    Else
        intFileID = ViewState("intFileID")
    End If
End Sub