Your question, can't be that agnostic, and must be divided in your two main concerns:
Transmitting passwords in the clear.
Xss, Sql injection.
No system will be declared secured per se, but you can try your best to minimize the risks by using proven concepts.
So let's say you have the chance to design your own "secure system", what do you need?
At minimum you will need a basic set of tools:
Client side data encripting: (Javascript here, i think you will find lots of info of how to send your data in data 64 or something like that, remember you are searching for one way or two way encription)
DB Encription: (One way-two way encription, but never save passwords in the clear)
SQl injection: (mysql_real_escape_string() comes to mind).
Every language has some sort of protection built in, it is when building large projects that sometimes we may forget to sanitize some querys
I repeat no system will ever be declared secure, however you can add some other security measures as in:
- access_tokens = timed-strings that allow to validate user login.
- captcha_after_few_intents = you should add this definitely.
- block_account_after_few_trys = pain in the * for users, but definitely worth it.
- login_token = store a token unique for that user, and use it in all GET/POST transactions
- SSL
From Bank Security:
- Automatic gsm devices that generates RANDS and access_tokens valid only for a short time.
- mouse keyboard: evitates keyloggers
- random_access_question:
- check_random_account_country_change = let's say user is from albany and next day he logs in from south america, that should maybe raise a flag to your system.
I'm sure you will find plenty of advice somewhere, but remember you at end will end talking html, and some js, your main defense is on the server side, so be good, or be good at it.