Off the top of my head, and some duplicates of earlier comments:
1. Tripod, tripod, tripod. Doesn't have to be tall but needs to be stable so you can leave it for a while. Good stiff rotational locks so it doesn't move once you set it.
2. Medium or high speed film. (200 ASA or 400 ASA) You'll have to live with more grain and less sharpness than daylight but will capture more stars.
3. Fixed-focal-length lens. That is, a 24mm wide-angle, not a 24~120 zoom set to 24mm. Two reasons for this: First, the zooms are heavy and tend to make the tripod slip. Second, most modern zooms are not par-focal and therefore don't have a true infinity stop, so you have to estimate focus.
4. Large aperture diameter (smaller f/number). A 35mm f/2 lens lets in four times as much light as a 35~105 f/4 zoom at 35mm.
5. Camera body with cable release (mechanical or electronic, either way) capability, manual focus and either 'B' or 'T' manual shutter speed settings. ('B' or "bulb" keeps the shutter open as long as you hold the button/release. 'T' opens with a push and closes with a second push or by tweaking the shutter speed dial...check your camera manual) If your camera has the 'T' setting, any cable release is ok. If it just has 'B', you're going to need a locking cable release.
6. Patience. Depending on how long of star trails you want to record, exposure times from minutes to hours are required. Remember the earth turns 15 degrees per hour, so that tells you how long it takes if you want star trails that cover 30 degrees of the sky.
7. Dark sky. Not just night, but dark like we love up in the mountains.
8. No moon. Well, at most, the moon behind you but even so, then you have to worry about the moonlit foreground for really long exposures (15 minutes and up).
So much for equipment. Now, setup and execution:
1. Find a stable, sheltered location for the tripod (less wind means less vibration) and set it up, just tall enough to clear whatever low-lying grass, brush, rocks, etc., would clutter your foreground.
2. Mount the camera, aim it and set the lens for infinity (turning off any autofocus) and wide-open (lowest number) aperture, say f/2, for example. Attach the cable release.
3. Take your shots starting at 30 sec and going 1, 2, 4, 8, 15, 30, 60 minutes or until you run out of patience.
4. Re-run the same sequence at a two-stops-smaller aperture (higher numeric value, say f/4 if you were using f/2 in the initial run in step 3).
5. Take the camera home, unload it, have the film souped and say a prayer to the deity or karma of your choice.
(If this sounds like a trial and error process, yup, until you know how the camera/lens/film combination you're using is REALLY going to work in the field.)
Gotchas:
1. Brightly-lit objects in the foreground...somebody in their tent with a bright light, for exmaple.
2. Bright campfire in the foreground.
3. Enough city lights to wash out part of the sky.
4. Reciprocity failure. Huh? Ok, film has both a "good" range of light (number of photons) that it likes to deal with, and, below that, funky things can happen. Below some threshold (photons/second), the electrochemical processes in the silver grains just won't happen because the electrons don't get hit often enough to be bumped up into the stable energy levels or something like that, so even though you might collect enough in absolute terms, the rate isn't high enough to expose the film. For most films, this starts at shutter speeds in the 2-10 second range, so you can see it is a potential problem. (That's why I said some trial/error is necessary for a given brand/speed/type of film.)
...hope this helps...
