All rules are meant to be mastered, then learn how to break them. The best rule for cropping, when shooting 35mm as your format, fill the frame in its entirety and crop tight to fit your needs.
There are other rules, such as with posing or the "Rule of Thirds" with composition that all go hand in hand with cropping. In all three of my books I cover these topics, my third book isn't out till the Fall however, but it will cover how cropping affects the pose.
Since there is so much when it comes to cropping, it's best to read articles or books, but the basics are simple, crop like you want the final photo in 35mm. With people, especially with glamour, always crop so your image is full-length, when not going for full-length, then crop three-quarters (above the knee caps or mid-thigh), and if you don't like that, then below the bust line, sometimes called "A button and half," think of counting down buttons from the collar. The last option is a headshot. Regardless, the rule when you start cropping people is "Above the Knee, or Above the Elbow, Otherwise you have an Amputee." Nothing against amputees, but the concept is crop where it appears more natural.
I hope that helps, here's some examples, enjoy, thanks, rg sends!
Full-length, though notice how the clothes crop too!
Three-quarter crop
Bust-up crop
Headshot crop
And the rule breakers, I like to call them "hybrid" crops
