It is important to hang both your thin-water presentation and approach decisions on a central tenant: observation. When you spot a trout in thin water, stop and watch what the fish is doing. Is it in a feeding lane? Is it cruising? What is it (likely) taking? What are its escape routes (so you're prepared for the fight)? What's the best casting position considering current and cover? What's the best cast (and perhaps mend) to deliver the fly properly? The answers can be the difference between success and failure.
Perhaps the most descriptive, yet succinct advice that I can give to anglers plying thin waters is this: be a heron. Herons ease into the water, watching and waiting, and then make a well-executed move when the time is right. So, make your moves deliberate and take your time when working the thin waters. Don’t rush in, even if tempted by a regularly working fish. If possible, stay low, and use any available cover to your advantage.
In addition to helping conceal you when it comes time to cast, cover can help a great deal as you move toward your casting position. When setting myself up, I try to use some piece of cover (a tree, bushes, fallen leaves on the water's surface, etc.) that blocks the fish's view of me as I make my move. And when I move, the motion is low, smooth, and even, with one eye on the fish's position, especially if the fish is moving. Losing track of the quarry has resulted in more than one spooked trout for me.
Cover is also very important to the fish. Look for any overhanging plants or under-cut areas as well as the edges of boulders, logs, weed-beds, and any other structure the fish can dive into at a moment's notice. In feeding mode, trout will park themselves under or right against the edges of such places, usually sipping daintily at passing food items. Indeed, one of the biggest brown trout I ever caught on Montana’s famed Armstrong’s Spring Creek was wedged in along some bank-side weeds, dimpling the surface like a chub.
In areas of slow flow or in stillwater environments, fish will actively cruise for food, tracing a feeding lane that is often defined by structure. Fish will travel up and back at fairly regular intervals in such lanes, allowing an angler to get into position and make a cast in anticipation of the fish's movements.
Barring the presence of acceptable structure, look for fish to hang in the shadows or along areas of elevation change. Fish in such an exposed environment can be almost unbelievably flighty, and the angler may need to place the fly ten feet or more from the fish to prevent spooking it.
Equipment also figures along the pathway to success in thin water. Everything about thin-water whispers "delicate" and "cautious", and your equipment should be tuned to those frequencies.
In some ways, the most important piece of equipment a thin-water fly fisher needs to have is a pair of polarized sunglasses. Precision is important in skinny water, and possibly seeing the fish is also a big part of the fun. Polarized lenses will also make it easier to see where you are wading, what the underwater environment is like (for fish-fighting purposes), and enhance the contrast between water and line.
As for leaders, reducing drag is often the number one consideration in thin waters, and those few feet of filament should do everything to further that end. This may mean leaders in the 10-15 foot range with butt diameters of .021" or less, ending in 5X, 6X or 7X. In stillwater environments you may wish to break from drag-reducing formulas and go right to a quick-handling six or seven foot leader that won’t hang up in the guides.
Thin-water trout can be challenging. If you hone your skills to match the conditions, however, you may find plentiful success. Indeed, thin-water skills can make you a better angler no matter what time of the year that you decide to wet a line.