I find the vacuum to be tremendously useful. The transverse abdominis (TVA) muscle acts as the body’s natural corset. A strong TVA will pull your tummy tight all the time, even when you are relaxed.
Do vacuums lying (with arms above head, perpendicular to the floor, pointing at ceiling), lying (with arms above head, parallel to the floor, pointing behind you), seated, and standing (this is in order of difficulty for me).
Additionally, think about why you are doing decline crunches. At the gym, they seem to be one of the most misunderstood exercises.
The reason to crunch on a decline is because it increases the range of motion (you can go beyond where the floor would stop you if you were on a flat surface)… it also adds resistance, but this is less important for now. Yet, I see everyone at the gym crunching on a STEEP incline, yet going down only a couple of degrees (not even to where the floor would stop them). If you will use a decline, go down ALL the way (to RIGHT before when your back will hit the bench).
The other tip that I have is to emphasize the eccentric portion of crunches. Instead of doing a million crunches, do 3 crunches where you lower yourself as slowly as possible.
Shoot for 3 crunches with 30 seconds of lowering, and explosive lifting (the “positive” or “eccentric” or “crunch” portion is as close to immediate as possible). If you can do 3, then increase the time to 40 seconds or add a rep, or something.
CAUTION: when doing eccentric-emphasized decline crunches, don’t put stress on your back by rounding your back. Keep your shoulders back and your neck in line with your spine at all times, even though this forces you to be working HARD with your abs. That is the point!
If you can’t do this cleanly, safely, and in control of your upper body, you should reduce the length of the lowering phase by a few seconds or do one fewer rep…
let me know how this goes!