Here’s a few exercises to get you started from Eric Horst:
The body curl. This exercise works the upper and lower abdominals, as well as the hip flexor muscles used in highstepping. While hanging straight-armed from a pull-up bar (or the largest holds on a hangboard), lift your knees to your chest, and then continue your upward motion until your knees pass between your arms. Strive for a slow, controlled motion throughout, especially while lowering your legs. Do six to 12 reps, then rest a few minutes and perform a second set.
The front lever. Introduced to climbing by the legendary John Gill, the front lever is the gold standard of core-muscle strength. Since front levers recruit all the core muscles in a highly specific way, expect this exercise to be difficult and possibly a bit demoralizing (most climbers can’t do a front lever).
Begin by hanging straight-armed from a bar or a set of Rock Rings. (Rock Rings are ideal, because they draw more of the upper-torso and shoulder muscles into play.) Pull up halfway, then push your hands forward, drop your head backwards, and lift your legs. Do all this in a single quick motion and attempt to position your entire body — head to toe — parallel to the ground. Squeeze tightly throughout your torso, buttocks, and legs, and attempt to hold this position for three seconds. It helps to think about pushing your hands towards your hips, even though you’ll be in a stationary position. You can make front levers easier by simply bending one leg, or by having a spotter hold your feet.
The goal is to hold the lever for three seconds, slowly lower to a hanging position, then immediately pull back into a front lever for another three seconds. Strive for three consecutive levers, then rest for a few minutes and perform one more set. It’s important to note that levers place a great deal of stress on your shoulders and elbows (just like steep climbing), so they are inappropriate for novice or out-of-shape climbers.
For less strenuous core exercises, like variations on sit-ups, crunches, and hip raises, check out this cool climbing website.