Night time can be the very best time to catch big fish, especially big brown trout and brook trout that may not ever feed other than at night. Here are a few things to remember when out after dark.
Use dry flies with a visible silhouette. This applies to the fish and to you. As visibility decreases go up in size on your dry flies and use flies that ride high on the water like large caddis, drakes, stimulators, and Wulff patterns. Also, position yourself to use the glare of the sunset or the moon to see the fly or at least the rise.
Try a little movement. Dead drifts are great and usually required for picky fish in the daytime but as darkness settles on the water a skated, skittered, or otherwise moving dry fly can entice vicious strikes.
Be safe. Remember where you can and can't cross heavy currents. Also, remember that a whole host of unsavory critters (skunks, snakes, and of course the more feared but probably less dangerous large creatures [read Sasquatch]) are nocturnal and can ruin your evening. Wear a headlamp or carry a flashlight for the trek back from the water.