There are a lot of exercises you can do.

One thats really helpful is like a sideways plank. It's a little hard to explain but it looks like this:

When you rest, just drop your hip to the floor. Put something under your arm because it can get a little painful.

Also, you could try doing [russian] twists with a medicine balls. Try just sitting on the floor with your legs bent in front of you and lean back just enough so you can balance your bent legs off the ground. Take the medicine ball, twist, and lightly touch it to the floor on your leftside, then twist and touch it to the right side and keep doing that for about a minute. If the medicine ball is tough, then do without for awhile.

Otherwise any sit up where you come up at an angle will be enough to get the transverse abs.

One good move I learned from yoga for the transverse abs is cross leg stretches. Start by laying on your back. Raise one leg so that it's perpendicular to the floor. Keeping it straight, and your hands to the sides of your body alongside your hips, gently and slowly cross your straight leg over to the other side of your body (R leg to L side) and you exhale. Keep your foot flexed and the heel reaching out. Take an inhale as the toes hover above the ground and then exhale your leg to center (use your hands for support, or bend your knees if it gets too difficult). Repeat 10 times each side with slow, deep and even breathing.

After this warm up, try both legs together. Love the heat!