hey.. im just a white belt so take what i say with a grain of salt.. but ive had some success with the arm-in and i used to suck at it. so i figured id just let you know my mistakes and you can see if they ring any bells. But for me, one mistake i used to make (and this might sound stupid.. but try it and see if it helps) I was actually getting the choke in "too deep".. whereas with the arm in triangle ive found getting a "shallow" choke in seems to be much more effective. what i mean by that is instead of clasping my hands on the same side of his arm (because i would reach all the way under his neck with my hand not holding the arm), i found it worked better if i would just place my wrist (of the arm going behind his head).. on his neck/under his chin, and then join hands there. another problem i had was i would lay back when i would do it. as if i were trying to arch my back or something.. i guess just thinking i could get more torque. but then they would end up getting my arm off and posturing up.. so now i like to stay crunched while i do it. then just like a rear naked, breathe deep, slowly squeeze, and make sure you keep there hips down. other than that the only thing i can think of is grip. i dont know the name of what i use.. but its similar to ball n socket except i keep my "ball" hand flat and grab my hand instead of wrist. but thats just me. im sure its different for different people. anyway. thats all i got :P