A strong core is the heart of a successful yoga practice, providing safe support and the foundation from which to move into more advanced poses. Take advantage of yoga's twofer: attention to your poses confers physical as well as spiritual benefits, and your abs are involved in almost every move. Performing asanas with attention will inevitably engage your abdominal muscles. Start to feel the tighten-and-release effect as you focus on your core in some go-to practice moves.
Cat-Cow
Cat pose and cow pose are often merged for a wonderful midsection stretch that unkinks your spine and lengthens your torso, front, and back. The combo massages your abdominal organs and gives your belly a good stretch. On hands and knees on the mat, lift your head and scoop your belly slowly for cow. Then smoothly reverse the move, dropping your head and rounding your spine for cat. Keep it slow and even, feeling a strong stretch at the peak of each pose. Cow pose releases tension from your stomach, spine, and groin. Engage your core for cat, pulling your belly button into your spine to increase the stretch and work the abdominal muscles.
Plank
Plank pose is a core-killer; you have to engage the abdominal muscles strongly to maintain the pose without sagging. Whether you are advanced enough to balance on your toes or modify the asana to rest on your knees, plank pose tones and strengthens your belly and chest muscles - not to mention the arms-of-iron it will give you.
One-Legged Pigeon
One-legged pigeon pose is a stretch, a stimulus, and almost a twist. You're bending your back while you stretch and stimulate the muscles of your abdomen and the front of your torso. You can do the pose with one thigh and your glutes on the floor and back leg extended. Alternatively, root your front foot and back bent knee firmly on the mat and grasp your raised back ankle with both hands to arch your body.
Boat
Ah, boat pose. It looks so simple but pushes you so hard. While you're balanced on your sitting bones and tailbone, your spine and extended legs forming a nice, neat "V," you are super-strengthening your abs, spine, and hip flexors. Engage your core to forestall the wobbles and boost that ab work just a little more. It's a "V," not a horseshoe.
Fish
You're supine on the mat, except for that extreme back-bending arch from your tailbone to the crown of your head. That's fish pose, and it's stretching and stimulating the organs and muscles of the belly, along with making your intercostals and hip flexors more supple so you can work your core even harder. Focus attention to your breathing deep into the belly in this pose. However, be aware of the tension in that abdominal stretch as well. You want an energized core to hold the pose comfortably and correctly - relaxed but alert, not sloppy, abs.
Standing Poses
Toned tummy muscles give you better posture and a more attractive profile when you're just standing around. Try some standing yoga poses to whip those abs into shape.
Triangle
Twist yourself into a glorious standing triangle pose and stimulate your abdominal organs as you activate and stretch your external and internal oblique muscles. Bliss. Don't forget to do both sides.
Half Moon
Tilt yourself into Half moon pose for better balance and improved digestion. Half moon does a lot more, though, including a serious strengthening of the abdomen - one of the main benefits - and increased power in the spine, thighs and glutes. You hold this pose powerfully from your core so it is important to start and stay engaged, abs continually working.
Warrior III
You can enter warrior III pose from warrior I or from a mountain pose into a lunge. While you're balanced on one foot with extended torso, arms and remaining leg held steady on a horizontal plane, your abs have to be crushing it. That means abdominal toning and core strengthening. Breathe evenly to help ground and stabilize yourself but keep that core engaged.
Lord of the Dance
Lord of the dance is just a cool pose. It mimics the iconic dancing Shiva of the Hindu pantheon, a graceful asana at once still and pulsing with energy. Natarajasana requires a strongly engaged core for stability and a really good stretch for form. So, you get stretched and strengthened abs while paying homage to one of the most powerful and playful gods in Eastern tradition.
Inversion
This marquee move works your abs hard as you get in and out of the pose and keeps them engaged as you balance upside down.
Headstand
You might not think of a headstand as an ab cruncher, but you'd be selling the asana short. The effort to raise both bent-knee legs to vertical overhead, extend the legs and hold them - or swing both extended legs up overhead - uses abs, core, and thigh muscles. Holding the position stable requires an engaged core. Balancing upside down is abs work. Sirsasana, or headstand, tones your abdominal organs while it strengthens the obliques, the rectus abdominis, and the transverse abdominis.
Navel-Gazing
When you check out which poses give an extra kick to your abs work, you increase the chances of envious yogis checking out your nicely toned abs. By engaging your core when a pose calls for that, deep belly breathing when you're doing pranayama, and tweaking a pose just a tad to get that slight abs edge, you'll work steadier on your mat, align yourself better throughout a sequence, and even stand a bit taller as your stronger core supports your spine in a more erect posture. Put some fire in your belly and work a few abs-friendly poses into your daily yoga sessions.