If you are using Membership
], then it is already there, as part of the Membership enabled Login control. If you aren't, then you are making life difficult for yourself at every stage of your website, and probably reducing security by a significant amount.
If you must home brew the security mechanism, then it isn't difficult to add "remember me" - all you have to do is add a cookie value which is the user id, then store it when the user logs in and read it when you load a page.
But I'd use Membership instead if I was you.