Fast GPUs will generate a lot of heat, and if they get too hot they will throttle by reducing their clock. You will not notice this unless you are monitoring clocks (which you can do e.g. with the nvidia-smi tool in Linux), but your machine will be slower. The GeForce 1080 cards are more sensitive to this since they draw more power than the 1070 cards.
F
irst, you want high airflow straight throw the case. Decide how the air will flow, put a bunch of case fans on the front to push it in, and then have the GPUs and existing case fans on the back push the air out (check the direction your extra fans blow whenmounting them…). As illustrated for the Corsair case we use for our quad-GPU machines, we have the air flow along the blue arrows, and then we try to block the holes at the top (marked with red) to avoid air flowing randomly. 
The first thing you need for this is a couple of extra fans. Fans with larger diameter are usually quieter, so pick either two 140mm fans or three 120mm. They are only a couple of dollars, so don’t be a cheapskate but buy quality.
Second, consumer GeForce GTX cards pull in air from the side of the card. This is not a problem when you only have a single card, but with multiple GPUs in the machine each extra card will reduce the air inflow to the previous card a bit. This is a bit worse in the latest-generation cards since there is usually a fancy black “backplate” mounted on the rear of card:
If you look carefully you will see that there is a thin line separating two halves of this backplate, and the entire reason for that is to make it possible to remove the left half in the image above to improve air flow in multi-GPU configuration. Don’t touch the four large screws – they hold the cooler for the GPU! Since we want all the air we can get, we remove both halves of the backplate to get it more naked:
This is all that we will do on the physical side. Once we get started with Linux there are another couple of tricks we’ll use. By default the drivers are optimized to make our office a sane place to work, so the fans will usually never max out, but there are ways we can force this to maximize cooling (but you probably don’t want to run it like that constantly if you are working in the same room).