Yep, lowering your body fat is the most important thing. You don't even have to have great abdominal development from them to show if your body fat is low enough. There are plenty of skinny dudes with abs just due to them having such low body fat percentages. Of course if your abs are more developed they'll look a lot better once you get your body fat down.
As far as working them out, here's what I recommend doing:
Warm up with roman chairs before every workout. Not necessarily enough to call it an ab workout, but enough to get the blood flowing. It's important to get the CNS warmed up before you start working out and roman chairs are a good way to do it. Also, if you're going to be training abs that day, roman chairs at the beginning of your workout will get your abs warmed up. On days that you don't train abs you can still do the roman chairs as part of your warm up (I average 5 minutes worth, not necessarily 5 minutes straight though) and not really have to worry about overtraining your abs. Just use your instincts here, you get the idea.
As far as specifically training abs, I usually train them 2-3 times per week and usually don't do any more than 100 total reps (you don't have to do hundreds and hundreds of crunches all the time to get good results). I may do a couple sets of crunches, then some reverse crunches on an incline (about 100 reps worth all together). It's important to not be fast and choppy with your movements; be under control, feel the muscles contracting and make every rep a deliberate one. Afterward I finish off with standing bent-over side twists (for obliques) using a light bar across my back - steady your pelvis and bend at the sides, squeezing at the extremes of the movement; this is a great obliques exercise because it keeps them tight and doesn't make them big. Try doing stomach vacuums after each set of abs for about 20 seconds!
Just train your abs when they feel "fresh," don't overdo it and make every rep count and you'll get better development if you stay consistent with it. Lower your body fat and you'll see more of what you've got.
Peace