At the recommendation of a very fishy friend, I have been experimenting with putting weights in front of my streamers or baitfish flies. There are some things in fly fishing that I just cannot explain. After awhile I give up trying to figure out why exactly they work, I just simply let the fish tell me they like it and leave it at that. This is one of those things. It must be something with the added jigging motion it gives the fly or maybe it is as simple as it helps the fly get deeper in the water column. But if you are having a slow day fishing streamers, try putting a weight on the leader above your fly. When streamer fishing, I often add one or two coneheads (the ones from your tying desk will work perfect) on my line. This allows them to slide freely on the leader but when you are fishing it, they will sit right above the knot. I can also control the amount of weight I want to use my adding multiple coneheads. When using a heavier rod for fish like pike or bass, I will even add a full on bullet weight like the one spin fishermen use above the fly. This is a pain to cast but gives it a deadly jigging motion during a slow retrieve. Never be afraid to mix up the way you rig and fish. You never know what you might discover!
More On Streamers:
I often use weights directly in front of a streamer, even though its less pleasant to cast, becauseI believe it makes the fly ride more natural. Unweighted streamer/leech flies ride hook bend down lower than the hook eye because the bend of the hook is the heaviest part. A moderate weight up front equalizes that to make it sink and ride horizontal and a heavy weight makes it jig. A vertical presentation is often deadly and requires weight at the head of the fly. Underbody lead wire, tungsten bead-heads, and barbell eyes all do the same thing but you can adjust the weight of the streamer with removable split-shot or tungsten beads threaded on the line before the knot is tied. Tungsten putty also works well, especially in leadfree zones like Yellowstone Park.