From James Kiefer:
[William Wilberforce] introduced his first anti-slavery motion in the House of Commons in 1788, in a three-and-a-half hour oration that concluded: "Sir, when we think of eternity and the future consequence of all human conduct, what is there in this life that shall make any man contradict the dictates of his conscience, the principles of justice and the law of God!"
The motion was defeated. Wilberforce brought it up again every year for eighteen years, until the slave trade was finally abolished on 25 March 1806. He continued the campaign against slavery itself, and the bill for the abolition of all slavery in British territories passed its crucial vote just four days before his death on 29 July 1833. A year later, on 31 July 1834, 800,000 slaves, chiefly in the British West Indies, were set free.
If you haven't seen the movie Amazing Grace, I recommend it. And I see at the website linked above that U.S. pastors, youth ministers, or school leaders can get a free copy of the DVD!