Michelle,
These are simply my suggestions from my experience so I am by no means an expert here!
But it sounds like you don't need to lose weight and quite possibly you could stand to gain some (?) Just a thought. But, for arm definition, I'll be honest, I personally work HARD and A LOT at it.
Meaning I don't just do upper body once or twice a week, I would say in some way every day I am working out my arms.
Let me clarify. I work out (now) 6x a week. I (now) start off with 45 minutes (sometimes 30 on cardio days) of cardio on the elliptical, followed then by my weight workout (3x a week) or additional cardio workout (3x a week) I also just added yoga so that tends to be an additional "workout" 2-3 times a week (some might not call it a workout and I dont count on yoga for my fitness goals but certain kinds do work your arms/legs HARD!!)
When I do cardio, it is often Cathe (now, I am new) or Taebo or Jillian Michaels; almost all of these perhaps with the exception of Cathe's step only workouts involve some type of arm work during the cardio; kickboxing especially. Then on cardio days I will sometimes end my workouts with mini-circuits, usually for shoulders. I'll use 5-8 pounds weights and do 3 sets or use the bands I have ~ anything to keep my heart rate up and work on chiseling out the arms.
Then of course on weight days I make sure my week is balanced out with lower and upper body work. I do a lot of pushups also.
***I don't know if this is the "answer" for you. AS you know we all have different metabolisms and I have simply continued to experiment and find out what works for me. I think the more you focus on it the more you will start to see results and that will encourage to do it more and it will be a great perpetual cycle. It worked for me!
:7