Moonchild,
As HudsonGray mentioned, feverfew is the most common herb used for preventing migraines. However, it should not be taken if you are taking any type of blood thinner, and it can cause mouth ulcers. I have read that eating the fresh leaves between slices of bread helps prevent the mouth ulcers (and would probably help with the bitter taste!). Research has shown feverfew to be very effective, but most people can't tolerate the taste or live in an area where it doesn't grow. I would suggest a tincture (made by steeping the herb in alcohol and water) if you have access to a health food store or pharmacy. A combination of tinctures would be even better:
10 ml of feverfew tincture
10 ml of valerian tincture
5 ml of lavender tincture
The valerian and lavender are mainly for relaxation; as I'm sure you've experienced, when a migraine comes on you tend to tense up, which makes things worse. I highly recommend Dr. James Duke's book, "The Green Pharmacy Herbal Handbook" for treating yourself. It lists helpful herbs but also the interactions/side effects they might cause. You could probably find this book at your local library.
Of course the feverfew capsules would be the simplest method. I would also recommend (for relaxation) skullcap or lemon balm capsules or tea. The lemon balm is delicious! The skullcap should not be taken if you are taking any form of sedative, as it has sedative properties.
Many Blessings from a Fellow Migraine Sufferer,
Bodhran