Be realistic. Even a camera that's only 30 years old can't be expected to perform like a modern camera. If you plan on using the old camera you purchase, make sure you go into it with the expectation that images may not as clear and accurately exposed as you are used to with a modern camera.
Lenses take pictures, not cameras. We sometimes forget that the lens is really what takes the picture. When you're looking at an old camera, pay particular attention to the condition of the glass. "Cleaning marks" aren't marks - they are scratches, and can have an affect on image quality. Unless the camera has a very simple single-element lens, the chances of you being able to clean the lens yourself is slim.
Check for leaks. The purpose of a camera body is to keep light from "leaking" in and spoiling the film. Old film doors, lens mounts and seams in the body may not seal as tightly as they used to. Older SLR's will often require a new "felt" kit to make them light-tight again. Many old folding and box cameras used primitive paper and wood construction that simply wasn't that reliable. Opening the film door and holding the front and sides of the camera up to a bright light can help reveal light leaks that could spoil a potentially wonderful shot.
Resist the urge to over-restore. With many old cameras, there's a strong urge to restore the camera to better-than-new condition - particularly if the camera has lots of brass and wood on it. Many antique collectors agree that the value of an antique can be damaged by any restoration attempt. That said, it's probably not unreasonable to do some clean-up on an older camera to make it operative again. You will need to use your own judgement here; if it's a relatively common and inexpensive camera, restore away! If it's a very rare or very old camera, speak to a professional first.
Know your films. Many old cameras use obsolete film sizes. One of my favorite old cameras to collect were the Kodak No. 2 folding cameras, because they use 120 film that is still readily available. 620 film is the same as 120 film; the only difference is the spool that it is wound onto. Larger sizes like 101 and 116 can be bought, but they are extremely expensive. Many of the cartridge formats are impossible to get now. With some of my large-film-format cameras, I would cut individual sheets of film and tape them to the film holder, but this is a tremendously laborious process that wouldn't be practical for extended use.
Have fun! Even if old cameras don't take pictures as well as their modern counterparts, there's still a lot of enjoyment to be had with old camera gear. There's a tremendous feeling of accomplishment when you see your first roll of film taken in a 70 year old folding camera. Enjoy the experience, and be sure to share your photos with others!