If it's feasible for you, I've found SSL peer certificate-based authentication to be a good solution.

I guess you can add all sorts of other authentication over the top of that (eg: passwords, session tokens, etc), but no passer-by is really going to be able to get past the SSL request authentication stage.

I use openssl + a perl script to generate my client certificates... the attributes are formated so that they're both human readable (by the customer's web-browser) and so that they contain a (secret) ID which my code can look up in the database.

-David