here is my method for generating abilities:
roll 4d6 then choose the highest 3 rolls and reroll the lowest at your discretion. the reroll MUST be kept. roll for each ability (6 rolls).
example: Drizzt rolls 4d6 he scores a 6, 2, 3, 3. he ditches the 2 and has a 6, 3, 3 left. feeling lucky Drizzt rerolls one of the 3's. he scores a 5! he now has a 6, 3, 5 giving him an ability score of 14. upon generating 6 scores Drizzt is now free to assign the scores to the abilities as he sees fit.
if instead of rerolling a 5 Drizzt rolled a 1 he would have to keep it giving him an ability score of 10.