Whether you're training your core to look good on the beach or to improve your performance, doing endless crunches, leg raises and half-hearted front and side planks isn't enough to see the sort of improvements you want.
Here are some factors I consider to be important when it comes to training the core:
1) If you want a good looking core, you need to drop bodyfat. There's no getting around this one, you could train abs for days but if you're not eating to support that goal, it's likely you won't achieve visible abs.
2) If you are exercising and doing 20+ rep sets of crunches, leg raises and exercises like that all the time, you need to switch it up. There are exercises I do with my clients that really gets the abs working hard and they'd struggle to hit 10 reps. It's all about training with intent, really challenge the core in the way it's meant to be challenged.
3) Something that caused a big shift in how I looked at training the core - the core is there to transfer force from the hips to the shoulders and vice versa with minimal energy leaks. This basically means keeping the core stable so power is not lost, this is very important for a lot of sports.
So to train it more fully, you need to look at exercises which cause the abs to resist movement, whether that is forward / backward flexion, side bending, rotational rather than using the abs to start a movement.
4) Prioritise your core training! it's a mistake I was once guilty of and see many people making the same mistake. Putting the ab training only at the end of a workout when you're already exhausted so they're not getting your full attention.
I like to cover some exercises as part of a warm up and use them during the workout, whether at the start, middle or end. The more of a priority it is, the more I load up the beginning of a workout with core training to really focus on them - You can train your abs at the beginning of a workout (just don't exhaust them) and still be fine to train with other lifts.
If you enjoy doing crunches, leg raises and all that, then keep doing it. However, if you want to start taking your ab training up a few levels there are better options out there which I can help with!
If you want some ideas of exercises to challenge the core, here is a little article I wrote, you're welcome to download the PDF and use it:
This is my, mostly, Personal Trainer musings and information which I hope you'll find helpful!