Adho Mukha Dandasana, aka Plank
How To Do It: Place the palms underneath the shoulders, spread through fingertips and engage your biceps and triceps to protect the elbow joint. Lift the knees and press back through the heels. Stack heels on top on toes. Lift the chin so that the neck is in line with the rest of the spine. To modify, lower the knees, untuck the toes, and create a straight line from your knees to the crown of your head.
Chaturanga Dandasana, aka Low Plank
How To Do It: From plank pose, lower the upper body inline with the triceps. The wrists should come directly below the elbows and the upper arms in line with the mat. Hug the elbows close to the body, press back through the heels and lengthen the crown of the head forward. To modify, lower the knees, untuck the toes, and create a straight line from knee to crown of the head. Repeat these two positions until you can really feel the burn.
Makarasana, aka Dolphin
How To Do It: From table top position (knees and elbows on the ground, spine neutral), place your elbows underneath your shoulders and your forearms flat on the mat, parallel to one another. Spread firmly through the palms. Tuck your toes under and reach your tailbone towards the sky. Gently draw the navel back towards the spine and up behind the lower two ribs. Role the shoulders away from the ears, and relax your head and neck. Hold for as long as you can - this is tougher than it looks!