Sites that involve credit cards or banking are in a whole different category. For those I think the best advice is to only use https bookmarks.

In that vein, here's an idea for a browser feature. When someone enters something into a form that looks like a credit card number, bank account number, or bank routing number, black out the entire browser (including the url bar) and require them to type in the domain they think they're submitting to. If they get it wrong they can't submit the form (at least for a few minutes).