The writings of Chris Townsend, who lives and backpacks in the Scottish Highlands, should be of considerable help.

Backpackinglight.com once published a truly excellent article on "Backpacking in Sustained Cold Rain" by Alan Dixon. It was published in 2006 (unfortunately BPL doesn't have articles of that quality any more), but now you have to pay $25 for a year's membership to be able to read it (you used only to have to pay $5 to read a single article). IMHO, the article is worth it.

A few suggestions from my own experience in the Pacific Northwest: Synthetic clothing (no cotton!). Go to whatever lengths are necessary to keep your insulation dry! Reserve a dry base layer and sleeping socks (synthetic) to wear only inside your sleeping bag, and keep them dry. Have a tent that you can pitch and take down without getting the inner layer wet. (That means one in which the fly pitches first). If your hiking clothing is wet (it probably will be), have a plastic bag to seal it in at night (it won't dry in your tent), and put the bag inside your sleeping bag. It won't dry, but at least it will be warm when you put it back on in the morning (and will dry faster from your body heat), and if the plastic bag is properly sealed, it won't wet out your sleeping bag insulation.