You need high levels of tension in order for your muscles to get stronger, and the stronger your muscles the more firm and defined they will become (Body fat will also make a huge difference to muscle definition, so get your diet sorted!). If we use the plank as an example, you are going to be able to create the most tension within the first 30 seconds, after that you will begin to lose tension and the benefits of the exercise.
If you can hold a solid plank, quads tensed, bum clenched and lats engaged (think about dragging your elbows toward your body) for 30 seconds, then for one, that's pretty good, but it is time to make it harder or switch to a more difficult exercise. Your first option is to raise one leg of the floor without shifting your hips to the side. Keep your hips parallel to the floor. If you can only hold it for 5 seconds, perfect! Stick with it and keep focusing on keeping all your muscles, not just the abdominal's tight. Another, slightly more difficult version is to raise one arm in front of you, still with full body tension and a straight body. The final variation, if you ever get there would be to raise one leg and the opposite arm.
Planks are a lot easier if you relax, you will be able to hold the position for much longer, but that defeats the point, you want to be focusing on the amount of tension you can generate, not the amount of time you can hold it for.
Holding exercises like the plank or doing high reps of any exercise will make you feel the burn and that must be good right? No, that burning sensation is just a build up of a waste product called lactic acid and it does diddly squat for your strength, or how firm your muscles are.
Other great ab exercises are the hanging leg raise, but if you don't have access to a pull up bar, try one of my favourite ever exercises, the dragon flag.
I don't usually do video's like this, because I find it strange talking to nobody. I'll never become a TV personality, but I get the point across.
You can train your abs, even when you're not training your abs. The double kettlebell front squat is an excellent example of this. Barbell front squats will also work, but there is something special about the kettlebell version. One of my favourite deadlift variations is the suitcase deadlift, where you position the bar at the side of you, then deadlift it one handed. It's great for your grip strength and your obliques will be screaming at you the day after.
I don't want to see 20 squats with 2, 4kg weights, remember how important tension is? Go heavy and aim for 3-5 reps.
Another great example of this is farmers walks, where you hold a weight in each hand and go for a wander. Again go heavy for 25 to 50 meters and tense those abs, not that you'll have much choice.
All the above exercises are isometric, meaning your abdominals are not moving, whilst they are working. This is great, but you should also include some dynamic (moving) work and no ab article would be complete without mentioning the ancient Turkish Get Up, as it includes everything we have just talked about. Try this variation where you pause for 5 seconds in each position, go heavy for 1 rep each hand for a total of 3 - 5 sets.