SMART SWAP: Swap crunches for planks. The pay-off: Lying on your back doing countless reps of crunches won't get you tight abs. To tighten your tummy, turn over and try planks. These work your entire core by isolating the muscles, getting you in shape - fast! To do: Get in position as shown in picture. Hold for 60 seconds. Repeat 3-5 times.