I like a lot of the suggestions especially the ones in the post above mine. However, some of the suggestions (in the thread, mostly avoided in the post above mine) actually hurt melee characters more than ranged characters! Having archer monsters do more damage just means that melee characters have to close more ground while taking more damage. If missile monsters damage is increased then have pure melee characters take less damage from missiles - not sure how the game would determine "pure" but it's a thought?
Another suggestion: monsters that absorbs your missiles until you kill it might be interesting. Perhaps a property of say a Death Ooze or other fancy jellies.
Personally I just think that PC missiles should do less damage. Especially the artifact missiles.