Despite it's limitations, I'm a big fan of allmusic.com myself. Generally, I hear of an artist that I'd like to check out, and I use their recommendations to decide which albums to start with. As you said, not foolproof, because sometimes a group's "definitive" work isn't the thing I'd like best, but it beats a pure random selection. There are other good published guides for a variety of genres--my Penguin guide to jazz is heavily dogeared, and I've still got the first edition of the Rolling Stone guide to rock. The key is to find ones that conform with your own tastes--and then recognize that no two tastes can ever be the same, so eventually you have to experiment on your own.
How do you decide on new music to buy?
What prompts you to buy new music, especially music from artists you're not familiar with? One of my favorite ways is to pick up on a thread or a review of an artist or genre that sounds interesting, and then head over to www.allmusic.com to read about them and similar bands. I often then buy a CD from the ones that sound most interesting and have received high ratings at that site. It isn't bullet-proof by a long shot, but I've found a lot of good music I wouldn't otherwise necessarily have found.
What's your strategy?