The basic idea is to have one sentence that sums up the cut and thrust of the article. I'll type out a few examples for you from Today's Times:
"Britain is doubling its aid to Pakistan to almost £500 million, reflecting concern over rising Islamic militancy in the lawless areas along the border with Afghanistan."
"The pregnant man who conceived a child after a gender-reassignment operation has apparently given birth to a healthy baby girl."
"Girls Aloud singer Cheryl Tweedy has died in hospital after being hit by a car in Oxford Street, London on Thursday."
"Britain's spending watchdog is refusing to approve the Treasury's accounts, compounding a miserable year for Alistair Darling, the Chancellor of the Exchequer."