Hey,
Cutting out a lot of the upper body exercises will (obviously) help. Since I knew that I was going to transition I stopped nearly all my weight training, push ups, pull ups, etc etc. Its slow going but after 7 months my arms have lost a lot of muscles, my back and shoulders are clearly much slimmer too. I occassionally do a short rep of squats just to keep toned but not muscled.
I've found lots of cardio helps, especially if you starve your body of the nutrients needed to promote muscle repair and growth, so that the muscles get their protein from themselves and this decreases their size.
x