Some of these seem patently ridiculous today. Pliny the Elder suggests cutting open a hairy spider to find two worms, which should be tied together, wrapped in deerskin, and attached to the woman’s body before sunrise. The many suggestions Soranus gives include squatting down immediately after intercourse, sneezing repeatedly, and drinking something cold. Some believed it possible to use willow or iron, both of which were thought of as sterile to prevent conception.
Other methods seem more likely to have worked. Some used what we now call the rhythm method could have, but how well it actually worked was probably affected by the prevailing belief that conception was most likely to occur at the end of menstruation. Women used various materials like soft wool dipped in various concoctions** as barriers to block off the os of the uterus, while wealthier men may have used goat bladders in the same way we use condoms now. Pomeroy also suggests that coitus interruptus may have been common enough that no one felt the need to mention it.
Some Roman physicians also felt the need to distinguish between contraceptives and abortives. Those who took the Hippocratic Oath swore never to perform any sort of abortion. Others, Soranus included, were willing to perform abortions because, they argued, the health of the mother was more important. Contraceptives, however, were perfectly acceptable.*** The wide variety of methods publically espoused tells us that much at least.
*According to Pliny the Elder, the last known stalk was given to Emperor Nero as a curiosity.
**Mostly to make them impermeable.
Hippocrates, On Generating Seed and the Nature of the Child 4-7, 13, 30 - Diotima
Pliny the Elder, Natural History 19.15, 29.27 - Perseus
Soranus Gynaecology 1.60-62 - Diotima
Pomeroy, Sarah. Goddesses, Whores, Wives, and Slaves: Women in Classical Antiquity. New York: Pantheon Books, 1995