Come on now, if you've worked out at all you know that building ab muscles and getting six pack abs isn't exactly the most challenging thing to do - although those oblique's are a pain in the beginning eh? Anyway, that part is all about the exercise routine; what is challenging is keeping them.
This is where most people screw up and ruin all their progress/work - you can't just continue to do your ab workout and expect them to stay! The number one thing you'll need to do is watch your diet; this doesn't mean you can't eat good tasting foods, it means you need to balance your diet a little better and eat the healthier alternatives.
You'll want your diet to mainly be high protein, moderate carbs and low fat/sugar - protein plays a big part in building muscle, too many carbs/too much fat will add body fat...
not something we're looking to do.
Next thing you'll want to do is keep your cardio routine up and going - too many people stop doing their cardio once they can see their abs nicely defined.
Apparently the mentality is: oh there they are...
cool *lies on the couch and begins eating nachos*.
You need to burn more calories than you consume to keep your body fat percentage down; a low body fat percentage = abs staying ripped and defined.
Not exactly rocket science right? Combining the two above with an effective workout routine will keep them abs for a lifetime baby - although you may want to change your workout every so often to avoid the "plateau" effect, you know what I'm talkin' about.
When something starts getting ridiculously easy, add weights or more tension.
Also remember that the abs are an endurance muscle, the slower you go the harder it gets.