Most core and ab workouts suck. They're filled with poorly selected exercises and executed with crappy technique.
Here are three ways to make your core workouts less sucky. All you need is a resistance band.
1. Cue a Canister Core
For core and ab work, you probably spend a lot time on your back. For these exercises to pay off, you need to do them better. That means less flaring of your ribs, less anterior pelvic tilt (APT), and less over-extending of the lower back. Put those poor positions together and it looks like an "X".
Instead, strive for a more canister-like position. That means your ribs and pelvis are more stacked, and your core muscles—whose job it is to keep them stacked—are more efficiently engaged.
This results in a stronger core, better posture, and less cranky low back and hips. Use a resistance band to help reinforce that position:
Band dead-bugs are one option:
Using your lats to pull the band down helps to reinforce the ribs-down position. You'll get a more canister-like bracing of your core and a far more effective set of dead-bugs.
Use these as part of your regular core training. Or, as an experiment, try a few sets before deadlifts.
2. Chase Activation
Hip flexors have gotten a bad rep recently. Some blame them for the cause of any back pain and frown at the thought of strengthening them. I disagree.
Strong hip flexors are part of an important chain of muscles that make up your core. Structural balance is important. If your hip flexors are weak, they need to be addressed. But yes, there are certain scenarios when you want to avoid working them, like when you're specifically trying to target the abs.
Activating your hamstrings reciprocally inhibits the hip flexors. By taking your hip flexors out of the equation a little during sit-up and crunch variations, you force your six-pack abs to work even harder. Specifically, 26-46% harder!
Instead of hooking your feet under something when you do crunches and sit-ups, try hooking them over. For example, dig your heels into the edge of a weightlifting platform or immoveable weight, or pull into a resistance band.
Pulling your heels in towards your butt will encourage more hamstrings activation which will lead to less hip flexor and more rectus abdominis (six-pack) activation. Try this deceptively difficult take on Janda sit-ups for all the proof you need.
3. Get Pummeled
Sometimes we just want to feel abdominal exercises more. Adding more reps or more resistance can work. The problem? The resistance we add is coming from the wrong direction.
To work your abs to their max, use exercises where you can "close the space between your ribs and pelvis." Pulling your ribs down towards your pelvis (like crunches) and pelvis up towards your ribs (like knee raises) will work to maximally shorten the rectus abdominis. Placing a band at the right angle during certain exercises will help load you in the right way.
Try this unconventional take on sprinter crunches:
The band works to add even more load at the top of the crunch where your abs are shortened. You'll definitely feel pummeled!