I used to do some commercial/institutional A/V installations. I agree with all that has been said, and offer these tips:
1) While cables are an excellent choice, I found that chain and quick-links of proper strength were easier to use, probably stronger than cables, and allowed for much simpler (though less precise) adjustment of hanging speakers. Buy plenty of chain and cut off the links you dont need; this also avoids screwing around with holding up a heavy speaker while you try to tighten the nuts on a cable clamp.
I recommend using the beefiest eye bolts you can find. Avoid eye screws, regardless of their size, unless that is the only option you have.
2) Keep in mind the orientation of the speaker. While your garage space is probably not overly acoustically sensitive (unless it's quite large), avoid the temptation to hang the speakers "sideways" if they use any kind of directional driver, i.e., a horn. It will make them sound strange, and you'll probably get some weird aural effects. Hang them in the same way you would set them on the floor, and then adjust their angle to your liking.
3) Realize that most speakers are not meant to be hung from a ceiling. Audio quality will suffer. I've installed bucks-up speakers in a high school gym, and guess what? Regardless of their quality, they still sound like speakers in a high school gym. Don't have unrealistic expectations, particularly if you are a bass fanatic.
4) All that said, tunes are an absolutely essential part of the garage experience. My personal gear at the moment consists of a floor-unit Sylvania turntable/8-track that I got at a consignment shop for $15. I replaced all the speakers with cheap-o radio shack stuff, and it sounds pretty awesome. It even has auxiliary RCA jacks (for the optional tape deck), which I use to plug in my iPod. Love it.
ANYWAY, best of luck with your speakers. Crank some tunes and crack a brew for me when you get them hung! Report back with how it goes!