How to set one page value to another page

Hello

I have label in first_page.aspx having value "Mar-2010". I want to set the value "Mar-2010" in dropdown in Second_page.aspx on page_Load.

If I am writing the code
if(!IsPostBack)
{
//my code
}
then it is not working. Otherwise, Outside if block it's working.
But it set value in the dropdown only, if writing outside the IF block.

I want to filter data of "Mar-2010" only."Mar-2010" shows in the dropdown but page shows all the value "Jan-2010","Feb-2010","Mar-2010" and so on.

Pls Help
Thanks in Advance