Arms and abs are definitely mirror muscles, but that doesn't mean we shouldn't train them hard! Because these muscles play such an important part the other lifts you do, training them is fundamental to your success in the gym. The work you do today can improve how you perform on compound lifts like the pull-up or bench press.
Abdominal exercises are most effective when you don't rely on your back, shoulders, or other body parts to help you do the work. A full exhalation activates those inner-most abdominal muscles like your diaphragm. By activating those muscles, your core is more stable and thus more capable of doing tough isolation work. Get all of your air out, and then initiate the movement.
Although it's great to initiate every movement by exhaling, don't forget to breathe. It's tough to train hard and build muscle if you're passed out on the floor.
Is your motivation wavering? Grab a buddy! Training partners can be a great way to add some accountability to your training regimen. Lifting with a partner can also help you push beyond your normal limits.