Also it helps to have some headspace in the keg. If you fill it to the weld then you'll not have much room for quick-carbing by agitation. Leave out another quart and the beer will have more co2 to accept.
I like to crouch with the chilled keg upright and between my knees and vigorously rock it back and forth with the co2 disconnected. About four blasts with the regulator set at max and the keg is carbed in less than 2 minutes.