Photographing lakes is a favorite subject for every landscape photographer. Think about one of those north american lakes surrounded by wild forests and snow capped mountains or some European lakes where picturesque and tiny villages reflect into. Probably, the mountain lake reflection is the image most landscape photographers aim to. For a successful photo with a mirror-like reflection, lake water must be perfectly still.
Try then to get to the lake very early in the morning when the usual daily breezes or occasional winds are not blowing yet. If there is any human activity on the lake like boats or canoes just wait a few minutes to let ripples dissipate. In case of mld breeze just stick around and wait for it to periodically die down. Avoid shooting with a super wide (zoom) lens as this tends to reduce the size of the distant mountains and also minimize the beauty of the epic landscape foreground.
If you decide to shoot a mountain lake scene without a polarizer filter sure you will get a nice mirror-like reflection but still your image will lack of some interesting underwater details. I suggest to take two pictures of the same scene (tripod needed in this case): one with the polarizer on and the other without. The photo shot with the polarizer filter on will give the maximum reflection while the other will unveil the underwater details instead. Once at home, just blend both your pictures with Photoshop in order to get the best mixture of lake reflection and water details.
When you shoot a lake scene always look for framing elements in the foreground. For example, try to get an appealing or pretty lake view framed by a window inside a building. A boat sailing gracefully in the water with a picturesque background made of mountains or a pretty lake village shot with a right composition and at the right time (golden hour is the best time often!) will create a beautiful scene and a winning photo!