One of my favorite exercises are planks. This includes the regular plank, the side plank, and their many variations.
Planks work your trunk stabilizing muscles, especially your abdominal muscles and your lower back. It strengthens them, but more importantly it improves their isometric strength and endurance. You'll be able to hold your "core" against resistance better and longer.
If you haven't, give them a try. Aim for 60 seconds in each position for the plank and side planks. If you can do 3 sets of those for 60 seconds each, make them harder - try raising one leg on the plank (alternate halfway through your time), or lifting one leg on the side plank.
Here is an excellent tutorial on the plank.