Youre 6’1" and 155 lbs., I doubt very highly that you have much fat covering anything on your body. The key to getting your abs to show more is to develop them more.
Crunches, leg raises, etc. are all great for working your abs, but nothing will tax your core muscles more than heavy compounds, particularly ones done in a standing position where your body has to hold itself upright without a bench or seat. Squats, deadlifts, and standing overhead presses, along with more ab-specific work will help to increase the thickness of your abdominal muscles, making them show up more.
The kicker to this is that those big movements will also help to build up the rest of your body, which, by the looks of your post in the rate my physique section, you need to do very badly.