Well, here's basically what I've got set-up on my two salmon fly rods...
I use a full floating line (SA Mastery Series Steelhead Taper for e.g.) cut back into the belly (approx 12 - 15 feet). I then attach a loop connector. Now I can use different types of tips of differing sinking rates. I have a number of Rio tips ranging from 109 - 166 grains of type 6 - 8 sinking rates. My most commonly used tips for the Vedder/Harrison/Chehalis/Squamish systems are the 109 grain Type 6 and 150 grain type 6. The larger the grain, the heavier the tip and the faster it will sink. Of course, the sink rate will also dictate how fast the line sinks. Anyway, this sink tip system is best used in flowing water. The floating portion of the line allows for proper mending and drift control. If you're not into making your own sink tip system, you can buy a multi-tip system (such as the Rio Versa-Tip or SA Multi-Tip systems which come with a pre-made floating line and tips of varying densities). The multi-tip systems are a bit pricey at first though; however, they do work very well.
I also have a reel spooled up with clear "slime" line which is good for still water (e.g. Harrison or pools) where the coho stack up before heading upstream. This is where the cast and retrieve fly works best. The clear line will help prevent spooking the fish, and coho are notorious for being very spooky. My slime line is also a very slow (intermediate) sinking line. You can increase the depth of the fly by either using weighted flies or waiting longer for the line to sink before retrieving.
As for coho flies - one of my personal favorites is the X-mas tree (I'll be tying up nine of these for the coho swap). This fly is basically made of flash material (flashabou, polar flash, etc.) with a bead head at the front. Colours can vary - from silver and gold to green, blue, red, and multicoloured variations. I've taken coho both on the swing/drift and retrieve with this fly. Other productive patterns are the rolled muddler (in varying colours, and with/without beads), coho blue, mickey finn, leeches, egg-sucking leech, wooley buggers (with/without beads), GP variations, and an infinite number of other patterns. I like to keep my patterns sparse. Also, I will not hesitate to go right down to 6 lb tippet when the water clears up.
One could write a whole book on the "art and science" of coho fly fishing. I can only guarantee you one thing - when you hook your first fish on the fly, you'll never forget it! Good luck and just ask if you want more advice.
