The classic text to refer to on this subject is the book by Hugh Falkus. In general I tend to follow his advice concentrating on dusk as the most likely catching time, but I do not fish through the night as he suggests. Wait until you hear them start splashing, and make plans for how to fish that spot. Keep everything simple. Use a single fly and a short leader as trying to sort out tackle problems at night can be difficult.
Position is critical to success.
You MUST be fishing over a place where salmon prefer to rest, unless you see a moving fish. As you must tackle-up in expectation of hooking a large fish, the leader strength etc must be suitable for this eventuality. I normally fish with a 16lb breaking strain leader. In order to get the fly down, in relatively fast moving water, a sink-tip flyline is almost essential. These have a poor name with stillwater anglers as they are reputed to be awkward to cast.