There are many guides on how to take pictures of the Milky Way from a technical standpoint, but fewer on how to set up for the perfect shot in advance. If you're looking for a good jumping-off point of the basic techniques of astrophotography, I can give a strong recommendation for Lonely Speck's guide on the topic.
14mm f/2.8, 38 seconds, 5000 ISO, foreground manually illuminated
There are several considerations you have to make in preparation for a Milky Way shoot. You won't always be able to shoot under optimal conditions, but a bare minimum needs to be met in order to get decent photos. Here are the variables:
- Moon phase (additionally, moonrise and moonset)
- Season
- Light Pollution
- Time of night (the Milky Way changes position throughout the night)
- Weather (clouds, humidity, 'transparency')
So let's break these down:
The Moon
One of the first considerations you need to make when trying to shoot the Milky Way is the moon. As we all know, the moon goes through phases. What this means in a practical sense, is that the level of illumination the moon gives in the night sky changes through the lunar cycle. Trying to shoot the Milky Way during a full moon is effectively impossible - anyone who's tried will tell you the moon lights up the sky like a second sun. You can use this website to see the percentage of the illumination. Anything under ~20% illumination is usable, anything over 50% makes it very difficult to get well exposed pictures.
The second factor to take into account with the moon is the moonset and moonrise times. If you're lucky, you can avoid the problem of moon illumination by taking your shots after moonset. You can use this website to find what time the moon is rising and setting, and where.
Overall, the week before and after a new moon is the best time for astrophotography. The day of the new moon is probably the best day to go shooting as the moon will not provide any illumination on that day.
14mm f/2.8, 30 seconds, ISO 5000
Light Pollution
Light pollution is another concern. The simplest way to check this is to simply check out a map that includes light pollution data, like this. My experience is that you can get a decent viewing of the Milky Way up until the dark green; past that and you will have difficulty making out any detail. As a general rule of thumb, if you're at least an hour outside any urban or metropolitan area, you should have a decent chance of getting some detail in the Milky Way.
One factor that's less often taken into account is airglow. This is essentially radiation in the upper atmosphere that can sometimes be seen over the course of long exposures. Another form of airglow can be seen when shooting towards areas of light pollution. So if you are shooting south, and there is a major city to your south, you may see some illumination on the horizon as a result. Make sure to take this into account when planning a trip or a specific composition.
Weather
An obvious factor is the weather. As you could guess, cloudy skies are not going to make for clear Milky Shots most of the time. The best website I've found for monitoring the weather is ClearDarkSky, which projects a forecast specifically designed for astronomy purposes. This can give you an hour-by-hour breakdown of expected conditions. I've used this website pretty extensively and had great success with its forecasts. Often you may be unable to find the exact area you plan to be shooting at, but anywhere within 20 miles will probably be sufficient.
Humidity is an often overlooked factor which can affect the transparency of the sky. In highly humid weather your shots may come out "hazy." While this can be recovered in post-processing (to an extent), it's best to plan ahead and avoid weather like this. Humidity can also cause condensation on the front lens element, ruining exposures.
The Milky Way "Season"
From the northern hemisphere, the best time of year to photograph the Milky Way stretches from March to October, with the summer months being the most ideal. Because of the Earth's rotation, by the end of October you may only have an hour-long window or less before the galactic center fades under the horizon. During the winter months, the galactic center may never even be visible as it stays under the horizon. The galactic center is the brightest, most contrast-y part of the Milky Way, making it the most popular focus. You can still get photos of other parts of the Milky Way throughout the winter, but they probably won't have the same punch as your pictures would in the summer.
Time of Night
Assuming good conditions, a final planning consideration should be what time you plan on shooting. This determines where the Milky Way will be in the sky and how much of the galactic center will be visible. If all the interesting foreground elements are in the south and you want to include the Milky Way in your shot, then it would be prudent to consider when the Milky Way will cross the sky at that point.
How do you do this? Fortunately you don't need much, if any, knowledge of astronomy.
By using a night sky program like Stellarium (which I personally recommend) you can input the rough coordinates of where you plan on shooting and it will output a visual simulation of what the sky will look like through that night (and day). This way, you can figure out where the Milky Way will be when you intend to shoot it. This is invaluable when it comes to planning a composition ahead of time.
Don't worry, you won't have any trouble finding the Milky Way when you get out there in person - on a clear day with no light pollution you can see the bands of the Milky Way clearly in the night sky.
14mm f/2.8, 35 seconds, 6400 ISO
Conclusion
As with all things, experience is what will teach you the most, but some proper planning can save you a lot of disappointment (take it from me, driving 4 hours only to realize the moon is too bright is no fun!) Every time I go out to take Milky Way pictures I make sure to check the moon phase, weather, light pollution (if the area is new), and do a quick run with Stellarium. If all signs point to go then I'm on my way!
The Milky Way can be one of the most beautiful, incredible things you can capture with your camera, and not as difficult as many people think it is. Hopefully you picked up some tips from reading this.
Happy shooting :)