Below is taken from "What really works". Sticking a sterilised pin into it gives a weak point for the thing to burst, when encouraged with a hot oatmeal poultice or similar.
Boils
If you suffer recurrent infections, you may need to take a long look at your lifestyle. Poor nutrition and an overload of stress will depress the immune system, making you more susceptible to an infection by the staphylococcus organism that is usually responsible for this skin problem.
Several potent herbs will act not only as blood cleansers, but as antibacterial agents to help prevent re-infection too. Cutting out stimulants such as tea, coffee, alcohol, sugar, sweets, and cakes will greatly relieve some of the burden on your immune system - so try and cut down on these if you can.
If you have an infection, bring the boil gently to a head using either pure tea tree oil, applied two or three times a day, or by making a hot poultice mixing calendula tea and slippery elm powder. (To make a poultice, mix the herbs with boiling water to form a paste. Spread the paste on clean cloth and, when it is no longer scalding, apply to the boil. Cover with a thick towel to keep the site hot.)