Read Nicki Doane's response:
The bandhas are an extremely important part of your asana practice. They relate to the subtle body: Mula Bandha (Root Lock), for example, is essentially the practice of containing your energy so that you don't leak the subtle life force that you are so earnestly cultivating in your asana practice. Its physical counterpart is the perineum muscle, which is located between the anus and the genitals and moves in an upward direction.
The bandhas are actually very difficult actions to make; most people tend to do them from a misguided physical place at first. Clenching the buttocks and sucking the lower belly in are not the correct actions—they should be much more subtle. I must say that, after 20 years of practice, I am just now starting to get a grasp on them.
This question takes me back to my first few years of practicing Ashtanga vinyasa yoga specifically. The concept of the bandhas and my actual experience of them were two very different things. Luckily, I had a teacher who told me that they take about 20 years to cultivate—and he was right. So, since we should be prepared for a lifetime of yoga practice, just be patient and they will come!