Fertility awareness for men is easy: just be aware that if you've got sperm, you're fertile, unless there's some medical problem we're not aware of.
It's a different story with women. Barring medical problems, women are only periodically fertile, basically once every cycle. Now I know that when we talk of these things we normally refer to a menstrual cycle, but let's think of it instead as a fertility cycle, thus emphasizing the changes in fertility, not blood flow.
The Biology
The first important thing to understand is that there is no such thing as a regular cycle. Cycles vary from between about 20 days to over 35 days, and an individual woman's cycle commonly varies in length. Variations of up to seven days length are perfectly normal for women who do not have their menstruation artificially regulated by the Pill or Depo Provera or some other hormone.
Just to pick a starting point, we'll say that a cycle begins at the end of a woman's period. At this time she is not fertile, and her cervix is actually physically blocked by a thick network of mucus filaments which form a "plug". Then, after a day or some days or as much as a week, her body starts to prepare for ovulation and (her biological imperatives hope) fertilization.
First, the pituitary gland begins to secrete follicle stimulating hormone (FSH), which stimulates the "ripening" of follicles in the ovary. As they "ripen", the follicles begin to produce increasing amounts of estrogen. As the estrogen levels rise, the endometrium thickens, the cervix becomes softer and more open, and the mucus "plug" is loosened. Thereafter, the cervical mucus begins to transform, becoming much more abundant and fluid. Filled with nourishing salts, sugars, and amino acids, the cervical mucus takes on a slippery, stretchy character. Its structure has actually changed from being a mess of criss-crossing fibers to a series of long threads lying side by side. The purpose of the mucus is to nourish any sperm that comes her way and to guide it toward the soon-to-be-released ovum.
Eventually, when the estrogen levels are high enough, the pituitary gland changes its tune and produces a surge of luteinizing hormone (LH); ovulation follows within 36 hours. The most mature follicle erupts, releasing an ovum, and we have ovulation. Then, LH causes the ruptured follicle to develop into the corpus luteum, which produces progesterone, which in turn causes the endometrium to soften in preparation (Mother Nature hopes) for the implantation of a fertilised ovum. The cervix becomes firmer and closed, while the cervical mucus again becomes hostile to sperm, acidic, thick, "plug"-like. If no fertilized ovum is implanted, the corpus luteum remains for around fourteen days, then it shrivels and dies; the level of progesterone falls; the endometrium is expelled (menstruation, to you and me); and so the cycle begins again.
The Experience
Every woman who doesn't take the Pill or Depo Provera or some other hormonal method of birth control experiences this cycle, though we may not have learned how to "read" the symptoms of fertility and infertility. There are a number of ways to observe this cycle firsthand. Some women take their basal body temperature daily, which you have to do at the same time every morning before moving around. I've never done this, so I can't speak about it. I follow my cycles by observing the changes in my (unfortunately named) cervical mucus. I learned rather quickly to recognize changes in my mucus, from the loosening and shedding of the thick, sticky "plug" into the watery, slippery fertile phase, to the abrupt cessation of mucus which signals that ovulation has occurred. Though other parts of my cycle vary by as much as five days, the period between ovulation and the onset of menstruation is an invariable fourteen days in my case. You might be different. And of course I correlate the mucus changes with other bodily changes: mood swings, swollen breasts, water retention, all that other fun stuff. These things just reconfirm what my body is telling me.
Some women, poor things, are afraid of touching themselves "down there" and so shy away from this whole concept in horror, but you don't have to stick your fingers into your vagina or anything to observe the mucus; it's easily observable when you wipe yourself after urination. Sometimes you're dry, sometimes you're very wet and slippery. It's natural, it's normal, and it's your body signalling to you where you are in your cycle.
And please note that fertility awareness is not the same as the rhythm method. The rhythm method is based on an abstract "regular" cycle that is read off a calendar. Fertility awareness is based on daily observation of the body in all its quirks and variations. Quite different, and much more effective.
Why I Know This
I learned all this because I didn't like my contraceptive choices. I didn't want to take an artificial hormone every day for 25 years or so, and besides wasn't much impressed with the yeast infections it gave me. The IUD made me bleed and cramp like hell. I couldn't place my diagphram properly (retroverted uterus or something like that). No one in my town sold cervical caps. Condoms were okay, but I wanted more control over things. I had a steady boyfriend who was willing to try a method that required us to refrain from intercourse for a good part of the time (and if you think that'll cramp your sex life you need to experiment more), so I started to chart my cycles according to the Billings' Ovulation Method, a form of natural family planning. Used as I was to having technology serve as contraception, I was pretty nervous about the whole thing for a good six months: it was all about me, about me knowing my body, understanding my cycle, deciding for myself when it was "safe" to have intercourse and when not. But after six months I'd charted it all accurately, it worked perfectly, and I was active, sexual, and - for the first time - happy about my fertility instead of seeing it as an enemy. That was 20 years ago, and I haven't used any other contraception except condoms since then. (I always use a condom with a new partner until we become serious and have HIV tests.) I haven't kept charts for years; it's become second nature to me to note where I am in my cycle based on the daily changes in my body.
While I have always used this information to avoid pregnancy, it can be used for the opposite purpose, if you desire. You'll be having intercourse at the time I'm avoiding it, is all.
For me, this has been the answer to the complaint that there are no good contraceptives, though of course it won't be everyone's.
To find out more about fertility awareness, try
www.fertilityuk.org/
www.alternativeparenting.com/preglaborbirth/natural_birth_control.htm
www.birchstudio.com/fac.htm
www.billings-centre.ab.ca/
And remember to let out a big belly laugh when they tell you to "avoid sexual intimacy" during fertile periods. Avoid intercourse, my dear friends, not sexual intimacy. It's not the same thing at all! | 
