Clear and Conspicuous
The second requirement, “reasonable provisions” will be upheld if there are no unusual, surprising, unfair or one-sided clauses that favor website owner more than they favor users. Potentially unreasonable terms are licenses to use users’ content, attorneys’ fees, waiver of trial in favor of arbitration, liquidated damages (fines), provisions about sharing visitors’ personal information with third parties or using it for marketing purposes.
Having a checkbox gives you strong protection but it’s not absolutely necessary to have the checkbox, as long as the “I agree…” sentence is somewhere near the button. This is an example of how Twitter does it on its account sign up screen: