It's a stupid tip, but the best fully-searchable archive you will ever have for your newsletter is your gmail archive. It's the only one that'll move with you seamlessly if you ever move to a different newsletter provider, and (currently, at least) Google's search is just better than anyone else's. Might be even worth setting up a standalone gmail account devoted purely to archiving your newsletter, if you might some day have colleagues who'd want to access the archive without getting into your personal gmail inbox.