Recently I seem to have been asked a lot about training abs. The reality is there is a place for direct abdominal training however it must be done within the right context. When using my training there is undoubtedly huge stress placed on the abdominal muscles, leading to stimulation. In many instances this can be enough - sometimes over training the mid-section can create a more "blocky" look as the muscles thicken. This needs to be considered, depending on the person in question and their structure/development.
Having said that, if you're looking to do abdominal specific training here are 3 things I recommend you consider.
Train Abs Fasted
My experience with abdominal training is that it's best done in a fasted state first thing where there's less food in the GI tract. This tends to allow for a better quality contraction and it reduces the chances of external influences causing failure i.e. bloating / bad digestion. It's also a good way to burn more calories in the morning after your cardio in this instance!
Be 3 Dimensional
Like every muscle you're training you must be three dimensional with your approach! Don't just do sit ups, crunches or leg raises. There should be a diverse range of exercises which hit all four areas of the core region. Not only is this better for aesthetics (when done right) it also helps prevent injuries, the last thing you want is imbalances throughout the core!
As with every exercise the key is to remain in control of the resistance to make the muscle work. With abdominal training breathing at the right time is absolutely critical - exhaling during the concentric in order to maximise the "squeeze" at the isometric point. If you don't get this right the effect of your abdominal exercises can diminish quickly