Stay indoors by the fireplace;- while factual, admittedly not relevant to the person trudging along a wet, muddy path through the falling rain trying to maintain a positive attitude about the wisdom of their decision to go hiking that day.
Living here in Oregon (aka the southern end of the Pacific rainforest) makes me agree with Ben's reply that it is not possible to both backpack and stay dry during multi-day, continuous rainfall. You will get wet; just accept it and go on, hunker down and stay put, or get out. The idea is to minimize the discomfort and to avoid hypothermia.
Until you experience it, it's hard to believe how much body heat (and how rapidly) you can lose to a cold rain continuously flowing over your raingear, sucking away your core heat. Add in wind chill and it's truly amazing.
I've been in a 40° wind-driven rain storm in the North Cascades where, even well-nourished and hydrated, and wearing a Gortex parka and pants plus a fleece mid layer and hiking uphill, I could not stay warm. This storm also drove the local hunters out of the woods, so I didn't object when one of them offerred me a ride back to town. That storm lasted three or four days.
A truly waterproof layer will keep the rain from reaching you (if properly seam sealed), but it will also keep most of the moisture produced by your body from escaping.