all that stuff he said ^ plus do it before flowering. also, don't just pull it upwards, that's still a lot of soil for the roots to hold on their own. i have someone help me flip it upside down while i have my hand flat on the soil, stem protruding through fingers and carefully slide it out; cut off that bottom half inch or just gently loosen up the bottom of the rootball and have the other person top up the pot, then flip it over and drop it back in. using a slightly larger pot (7g?) will pro'ly make it easier.