He announced that cutting troop numbers was now 'the basis of our strategy'.
Mr Brown spoke as two more soldiers' bodies were flown home for burial. So far 212 soldiers have died in the conflict.
He described his strategy as ' Afghanisation' - a grim echo of how Richard Nixon described the policy in Vietnam when that conflict was turning into a quagmire that ended in humiliating defeat.
Mr Brown used the word four times in his speech to the International Institute of Strategic Studies think-tank in London.
He said: 'Ours is a four-pronged strategy for accelerating the Afghanisation of the campaign. First we will partner a growing Afghan army presence in central Helmand. Second we will strengthen the civilian-military partnership, including on policing.
'Third, we will support the governor of Helmand by strengthening district government and a more effective, cleaner government in Kabul. And fourth we will build on the success of the "wheat not heroin" initiative to wean farmers from drugs crops.'