Successful late summer fly fishing patterns should not be centered on the same methodology that you use for spring trout fly fishing. The best fly patterns for late summer have to be the terrestrial group (Grasshoppers, Ants and Beetles) and smaller nymphs (Zugbugs, Hares Ear's and Pheasant tails) but how and when to fish these patterns is just important as what the flies actually look like.
The first key point to remember is that all streams and rivers are not created equal. Spring creeks are ideal for trout fishermen who don't have a ton of free time to get out fishing. These bodies of waters stay at a constant temperature meaning trout can live and feed anywhere at any time. In a freestone streams it's is not so easy, water temperatures fluctuate causing fish to change their respected patterns. If the water in the river gets too warm the trout will be forced to move upstream seeking colder waters. If the stream gets so warm that the trout cannot find colder water then they will leave the stream completely and reside in a tributary or spring entering the stream until the water returns to a suitable temperature.
This water temperature also affects a trout's feeding habits. In most cases, trout do not feed much in extremely warm water. If you have an extremely hot week, fish will be lethargic and hard to interest in what you have to offer. If you have to fish during this type of weather, try doing it really early in the morning (4-7 a.m.) or right before dark when the temperatures are not so hot. A good point to remember is that with these hot types of conditions fish may only feed for a short period of time. Just because you catch one or two fish, don't expect the action to be as consistent as the rest of the year. Another option is to key in on the deepest of holes on the stretch of river. Deep holes will have colder water on the bottom and you might be able to find a few active fish laying deep.
If you do happen to find some cooler water temperatures and feeding fish, then the next important factor to think about is water depth and clarity. During these hot months many streams have very low water depth (and are crystal clear) making presenting flies to spooky trout difficult. A general rule of thumb is that fly anglers should approach every hole or run from the bottom up very carefully. Sneaking on your knees or hiding behind vegetation while casting will make a huge difference to the numbers of fish you see. It is also a good idea to use long leaders this time of year. The longer leader you can use the better. In low clear water conditions fly lines can and will spook fish if they get too close to trout.
The hot summer conditions don't have to put an end to your fly fishing adventures. Get out early, be stealthy find the cooler water temps and fish will be receptive.