For me it was grip and core strength, i found exercises like heavy swings and OLifts, Kipping Pullups, kettlebell snathces and especially kettlebell flips and juggling. With grip i work towel pullups, COC grippers/super gripper. Overhead squats i find especially good for flexability against figure fours. For gassing i find doing CF as Rx does the trick no worries. I mostly do no-gi.
I havent tried it yet because i dont have one, but having a roll with a weighted vest on might be benificial.
I know Steve Maxwell has a dvd available that is specific for BJJ athletes. Has any one seen it ?