There is no secret to having your abs show. In order for them to show you need to have low bodyfat. 18% is much too high for them to be showing. TO achieve the 6-pack look, you need to aim lower then 13% bodyfat. Around 13% is when you will start seeing them, and as you get lower, the abs will show even more.
Start doing more cardio, and eat better if you want a six pack
If you want to have a nice etimate on how much fat you need to lose based on the information that you gave me, here it is:
your LBM is roughly 158.26 (this is with no fat on your body). Your goal bodyfat will be 13%. The formula to figure out the weight you will be if you lost ONLY fat would be LBM / 1.0 - %BF goal= weight in pounds.
Therefore after losing fat only, you will be about 182 lbs and 13% BF before you really start seeing those abs nicely.
Hope that helps.
Good luck to you!
*EDIT* Note: Your abs are like any other muscle on your body. If you want them to look more defined (remember, they will still only show when you have low BF%) then you should be incorporating weighted crunches into your current ab workout. You also should be careful not to overtrain them. I do abs 2 times a week and completely kill them when I do them. Don't do more then 2 intense ab workouts a week. When it comes to bodybuilding, too much is not always a good thing.