If you're really serious about hearing conservation, these are what you want. Basically ear buds that you wire into your headset/helmet surrounded by ear plug foam that you then wear under your headset/helmet. If you're really serious, you can get custom molded ones.

Communication Earplug Kit

They work really well and allow you to have the dual-layer protection from ambient noise (plugs and cups) and still let you hear the ICS/Radio/Warnings. If you're doing long hours in a SE piston or turboprop, I'd say it's highly recommended, probably overkill if you're in a big multi turbofan.