Parallel bars are a classic piece of gymnastic equipment, but they shouldn't be limited to gymnasts. There are many parallel bar exercises that the average fitness enthusiast can perform to increase core strength, coordination and muscular endurance. It's a good idea to work with a partner as you start parallel bar training so you can spot each other. Padded floors or mats are also necessary, particularly when you're starting out and any time that you start working on a new skill.
The support position is essentially the extension phase of a dip, with your body vertical and both arms straight supporting the torso above the bars. It seems simple at first, and it is, but the core stabilization required for a solid support is essential for every other exercise you'll perform on the bars. Give your support workout a boost with support swings, in which you'll swing your body back and forth, legs and arms held straight and torso taught. Use a spotter with swings, as it's easy to fatigue and lose your hold. Throw in some dips on the bars to build upper torso and triceps strength, as well.
Upper Arm Swings
Once you've built the core strength needed to hold a support and have tried a few support swings, move to upper arm swings. The idea is similar to a support swing, except the medial or interior aspect of your upper arms will hold your body weight. Start by hanging with each bar seated in the middle of your upper arm across the bulk of your biceps and triceps muscles. Keep your forearms at a 90 degree angle or so from your upper arms--hold the bars loosely in your hands if you feel the urge to bend your arms inward. Start your swing with a few small kips, then keep your body taught. Keep your shoulders above the bar and continue the swing with your core muscles.
Once you've developed a good support, start working on your L-sit. This is essentially a support position with your legs held straight out from your body, forming an L with your torso. An L-sit requires great strength from your abdominal muscles, obliques and hip flexors. Work up to holding an L-sit for 30 seconds. Start by doing simple leg raises while in a support position--bend your knees if you can't hold them straight. Then begin static holds. Try to hold a full L-sit even for a few seconds at a time. If you can't quite get it, tuck one or both knees into your chest to lessen the load on your abs.
A full planche is an impressive show. It requires the gymnast to hold his body straight-armed over the bars, body taught and parallel to the ground. Planches are advanced skills, but beginners can start working on them with ball planches. Start in a support position and tuck your knees tight into your chest. Lean forward over the bars, keeping your arms straight, and tighten your back to raise your hips. A good ball planche will place your back parallel to the ground, shoulders well in front of your hands and arms at an angle over the bars. This is another exercise that should be performed with a spotter.