Usability/legal - What is the best way to handle terms & conditions in a form?

I want users to accept my terms & conditions before signing up for my service, in a way that is userfriendly but that is also legal and stands in court. Different websites handle accepting terms differently:

Facebook doesn't have a checkbox at all. It's homepage says: By clicking Sign Up, you agree to our Terms and that you have read and understand our Data Use Policy.

Twitter doesn't let you check a box either, but shows their full policy on their signup page: https://twitter.com/signup

Google makes users check a box and links to their policy, but doesn't show them on the page. https://accounts.google.com/SignUp?service=mail&continue=https%3A%2F%2Fmail.google.com%2Fmail%2F&ltmpl=default

• How do you handle terms & conditions, and why?

• Also, if you have a checkbox, do you have it automatically checked or not?

• Do you know of any examples where terms were considered not legally binding because there was no checkbox?