Change Your Thinking:
It takes a bit more skill to catch Lake Trout, especially in the summer-time. For years people have been using trolling rods with thick wire line or downriggers to fish deep. It's the exact opposite ideology if you want to catch lots of trout. Thick heavy line causes more friction with the water, thus it is harder to go deep. Thick line is too visible and downriggers scream through the water spooking the fish, thus you would only catch 1/5 the amount of fish compared to the method below. Plus heavy trolling rods are not sensitive enough to feel a small fish hit your lure when you are fishing 60 feet deep.
You need a light action rod with six pound test line. You also need three-way swivels and a 2 or 3 oz. weight.
By using light line, the line has less friction with the water and slices through so that your line goes down to the bottom without having lots of line out. Tie two 4 foot pieces of line to your three-way swivel. Use a 2 or 3 oz. weight on one line and a light lure on the other. Lake Trout like small lures. Use #1 or #0 Mepps or Blue Foxes. The absolute best lure for Lake Trout is the Sutton Silver Spoon. Try to find a 2 inch weightless. Your local bait store will have to order them for you. It's very rare to see them on the shelf.
You only want to move just fast enough for your lure to work and no faster. If your boat is moving too fast, it will be very hard to find the bottom of the lake. If you are using a boat with a bigger motor and it's hard to keep slow, try back trolling.
Finding the bottom:
The most important aspect of Lake Trout fishing is letting out line to get to the bottom. DO NOT JUST LET YOUR LINE OUT UNTIL IT HITS BOTTOM. Hold the rod in one hand with the bail open. Let the line run through the palm of your other hand and grip the line. Once the boat starts moving and you have a good straight troll going, open your hand with the line then close it again. This way you can let out a foot or two of line at a time. Get a rhythm going. Open, close, open, close. Your rod tip will bounce up and down as you release little bits of line at a time. The rhythm of your rod tip bouncing will be disrupted when your weight hits the bottom of the lake. When this happens, reel up a foot or two. The purpose of this procedure is to keep your three way swivel setup from getting tangled.
Trout are funny when it comes to hitting your lure. Small ones will hit and then take off so you know you have a fish on. The really big trout will hit the lure and slowly swim away. They are so big they don't know they're hooked. So if you get a snag, make sure it's not a fish before you start toughing on your line. If it's a big trout, loosen the drag on your reel because they will go nuts and strip a 100 yards of line off your reel before you can turn them.
In the summer time, Lake Trout hit best in the morning between first light and 10:30 AM. They will hit better if the surface of the water is dead calm and it's a clear sky with high pressure. Any other conditions will cause them to slow down. If it's early spring, the trout seem to feed in other parts of the day, thus they are easier to catch. In some lakes the trout feed before dark.
Structure and wind:
Take a close look at the structure of the shoreline and try to extend the elevation patterns into the lake. If you see a cliff, odds are the water is deep at it's face. If you see a string of islands, odds are there is a shallow shoal that runs between them. Trout like drop-offs so you would want to troll parallel to the string of shoals and not over them.
When you drop your line to the bottom, count how many times you let out line. You can get a good estimate of the depth. Try to stay in 40 to 60 feet of water. If you come across a spot and catch a trout, odds are there are more of them there. The wind is very important when trout fishing. Traditionally for warm water fish like Walleye or Musky, you would fish on the side of the lake were the wind is blowing. The logic being that the fish follow the surface food that is being blown in. With trout it is the exact opposite. The wind also blows the warm surface water which does not hold enough oxygen for the trout. Thus fish the side of the lake where the wind is coming from.
Depth: Just after ice-out --> Between 10 feet and the surface
Mid Spring --> About 35 to 45 feet deep
Late Spring --> About 50 to 65 feet deep
Summer --> Summer is the tricky part. Many believe that the Lake Trout go to the deepest part of the lake and stay dormant. In actual fact, the Lake Trout stay suspended in 53° thermal layers. Why are they there? That's where all the bait fish are. Lake Trout feed on White Fish and Suckers which they find suspended in schools. There will be trout on the bottom but they are not feeding. When they do feed, they come shallower to feed on suspended bait fish.
In the Spring, the Lake Trout will be right up to the surface. As the water starts to warm up with the changing weather, the trout start to go deeper. Here is the approximate depth for different times of year. This is not true for all lakes. Some smaller spring fed lakes will have Lake Trout shallow all year.
It's good to have a depth finder so you can map the schools of bait fish that are suspended. When you do come across a school, troll around the outside of the school. The Lake Trout sit right underneath the school waiting for weak or injured fish to venture outside the school. Out in the middle of the lake, you will find these schools of bait fish in the 40 to 60 foot range. It's different on most lakes but this is a good place to start.