Encoding and encrypting cookie characters helps slow down cookie interlopers seeking sensitive material such as credit card numbers. Paul McFedries covers the basics here and gives you an algorithm to use in encryption.
Forms generally require the user to fill in at least one field. Paul McFedries shows you some ways to encourage users to fill in mandatory fields.
Cross-browser scripting might be difficult, but it is possible. Paul McFedries gives you some pointers on scripting for multiple object models.