A similar
herbal contraceptive measure is the practice of
placing a sprig of parsley in one's vagina. This is done during the
premenstrual phase of a woman's cycle. The
parsley, like Queen Anne's Lace, will release oils that make the
endometrium less "sticky", and soften any
mucous plug that begins to develop immediately after
conception, if it has occurred.
Parsley can also be consumed for the same purposes. I recommend brewing a tea with the leaves and honey to avoid eating the plant, which can become quite bitter in quantity.
The body is far more likely to go ahead and menstruate on schedule after this.
The parsley method is certainly not failsafe, and pushes the grey line between birth control and abortificant, since one waits as long as a few weeks after a possible fertilization to employ it.
A second method to try is vitamin C*. As ambitious as happy little sperms are, they need an alkaline environment in which to live, and 6-10 g of ascorbic acid per day near and after ovulation is enough to make the uterus too acidic for implantation to occur. It will also reduce the production of progesterone, the purposes of which are outlined above.
Vitamin C tablets may also be placed in the vagina instead of ingested. You can do this either immediately after sex (1000 mg a day for 3 days) as an emergency procedure, or beforehand (with enough time to dissolve) as a preventative one. Use of vitamin C as a spermicide can be very irritating to the delicate skin up in there, so soothe with aloe vera after use.
*Without bioflavinoids, if you can find it.
Above all, know your cycle well. These are not fully safe alternatives to protected sex, even if you aren't supposed to be fertile. And in the off chance of being exposed to an STD, use a condom, fool.