I have felt like Cruella in the past few weeks during class but I think there's a lot of bad plank technique being overlooked by instructors. Here's the problem - you're probably not even aware that you're putting your back or hips into awkward positions because you've trained your body to think that's normal. Like I say in class, if you are not 100% sure you're doing it correctly, check your form out in the side mirror, ask a personal trainer to look at your technique, or come up to me after or before class and chat to me about it! There is NO shame in starting on your knees and building up - that is so much more impressive than doing the "belly flop" or "butt in the air" look.
Here is a guide to your perfect plank and tips I like to use when doing my own abs training:
I once listened to an expert trainer describe core strength as being like a spiderweb. Your belly button, when pulled in towards the spine, is like the centre of the web or the basis for your strength. Once you are strong here, that strength supports the rest of your movements. That means, supporting great posture and alignment when you're standing, walking or sitting. That means, enabling greater strength and technique when you are squatting 100kg or chest pressing 20kg. Try to visualise that spiderweb when you pull your bellybutton in and brace.

• Align your elbows under your shoulders and keep your gaze ahead on the ground. Your neck should be in line with your spine
• Start on your knees. If you're ready, raise into full plank with your hips lifted in line with your shoulders and heels pressing back
• Imagine someone is going to kick you in the abs: brace!
• Relax your shoulders back and down your spine so that they are not rounding up into a hunch - I see this a lot!
• If you need to raise your hips up to maintain your plank, stop. You have taken the focus out of the abs and it's time to retrain your body. Lower your knees and start building from here.

• Reverse plank - the same rules apply. Keep your hips lifted and your abs braced
• It is hard enough without raising a leg so only do this if your technique is PERFECT!
• Keep your gaze on your toes and your shoulders away from your ears - no hunching
• Point your toes and take rests between sets