A great way to tone your core is to attack every aspect of it (abs, hips, lower back, and butt). Challenge all of these areas in one session and you'll gain leaned muscle.
Your core is interconnected so if any of these areas is weak it will make your stomach poof out.
I built my workout (Core Blast) from exercises I found here and at ab-workouts-guide.com its designed to hit all of the areas listed above. If you like it checkout Core Abs & Stretches, Autumn Abs, Drill Sergeant Mom, Extreme Plank Pyramid Challenge, Mid-Section Drill, Multi-Plank Challenge, P90x: Ab Ripper X, or Core & Glute Kicks they're fun and challenging.
Also don't forget to do plenty of cardio. Melting fat off is one of best ways to get your abs to show. I usually tell people that a good rule of thumb is to get 30 minutes of cardio (more or less depending on the intensity).
Good luck and God Bless!