I'm the opposite to Mike, fishing single fly as often as he fishes multiple flies. Yes you can get multiple hook ups, it has happened to me twice in the last 20 years.
Traditional loch style fishing, the majority of what I do, is usually done with a three fly set up. Two flies are sometimes used and four not unknown. Three is usual though. These are most often wet flies and the total length from fly line to point fly can be up to 25 feet. This is dependant on the conditions, generally you should use as long a leader as you can manage. This is an aside though, we are talking about dry and nymph or wet fly set ups.
To illustrate I have done a quick sketch of the various set ups I have used. The flies and tippet are the variations here not the leader. Therefore I've only shown the variations for the fly and tippet. The flies I've drawn are just to illustrate that "there would be a fly here", not an indication that a particular set up suits a particular fly. Also I have presumed that the indicator dry fly will be closest to the fly line. I know that this is not always the case, but it is what I use most of the time.
Various leader setups.jpg 52KB
Of the three the one I use most is the New Zealand set up where a length of tippet is tied directly to the bend of the indicator fly. There is one overriding reason. Its the simplest. I like simple, simple I can do. It is very easy to form the knot as if tying the tippet onto your finger, then slip it onto the hook and tighten.
I've tried Colin's method of tying a loop in to the fly to hang a length of tippet off. It means having a special set of flies just to use with this method. Also I find it fiddly to adjust on the water.
The dropper method I will use if I feel that I might want to change and have the dry on the point. Sometimes a useful technique, but it is more difficult to cast.
Two in the eye I only use to overcome the doubts of people who think the trailing tippet may interfere with the hooking of fish.
There are, undoubtedly, other ways to set this up, these are the ones I've tried.