If there's no time to hit the Pilates mat, then learn to engage your abs during other workouts. It's really simple to do, and all you have to think about is initiating your core (which really, you should be doing anyway!). The next time you find yourself at Zumba, on the treadmill, or in the weight room, learn the perfect time to start exhaling toward washboard abs.
Zumba: In this class and other dance-inspired workouts, think about your abs guiding the movement behind your hips. With each pivot and side step, lead with your core so that you utilize it for the whole hour. Stronger abdominals will take the pressure off your back, giving you that tall and effortless posture all dancers hope to achieve.
Barre: Although these classes already include an abdominal section, there are tons of other tricky ways to squeeze an ab workout in between the curls. Weights and thigh work are prime times to start working your core, not just because these exercises take up a large majority of the hour, but because engaging your abs can take your mind off the pain in other areas, as well.
Weight lifting: Think about contracting your abs in when the weights move one direction, then keeping them in as you inhale the other direction. You want to think about keeping your abs where you want them, which is in. By holding the contraction in your core, you can strengthen those muscles up to twice as much, gradually making them stronger.
Cycling: In cycling, you need to engage your abs on the fast runs, but the breaks in between are key to keeping a strong midsection. That time you get to lower the resistance and reset your stance is the same time you should focus on your abs. Since you don't need to focus on your feet tripping you up, use this "downtime" to initiate your abs the most. By the time you're ready to go back uphill, your core will already be warmed up, resulting in better posture.
Running: Using your abs during each stride is actually a principle of Chi running. The form of running, which blends the principles of T'ai Chi and forward locomotion, focuses on posture. Part of aligning your body means engaging your lower abs, so this action can help you make more efficient use of your energy while sculpting your midriff.