We asked product developers and a slew of testers to check out the latest DIY keratin treatments that claim to smooth and straighten hair for up to 30 days. The consensus: They really work! Even better, since they don't contain formaldehyde (or any of its derivatives), they're not as damaging as many of the first-generation versions, says cosmetic chemist Jim Hammer. Keep in mind that some still rely on harsh chemicals to straighten strands, so be sure to pick the right formula for your hair type and always apply it in a well-ventilated area.