I use Alpine MusicSafe Pro attenuators, which are awesome. They're really small, dirt cheap (£16) and they have a good flat response, literally allowing me to monitor and hear as usual, and only stopping it from killing me when some tool yells a request in my ear (which is usually what i find kills my hearing the most!) I'd definitely recommend those.

They also come with 3 different filters, so i can change them depending on how loud the situation is...