Bars: Front Hip Circles
One unique feature to the front hip circle is that it is not highly dependent on strength. While it certainly makes it easier if you're buff and strong, not understanding the correct timing will undermine your efforts.
Nonetheless, good strength is always helpful. The most important group is the stomach muscles, which you will need to "snap" your body around the bar and to maintain the piked position of the skill.
Now, the crucial part: timing. To correctly do a front hip circle, you need to keep your body extended as you lean forward until you are about to reach horizontal. Now is the time to put your stomach muscles in action. As quickly as you can, snap your body from the extended position to a pike. (Many beginning gymnasts exaggerate this exteded-to-pike snap -- and this is okay -- but as you become more comfortable with the skill, you can start from a hollow body position and go to the pike.)
Be sure that you think about bringing your chest to your legs, rather than letting your legs drop as you bend your body. Making this mistake will result in a decrease in how much power you get going around the bar.
Very often, gymnasts learning the front hip don't "finish" the trick. They snap their bodies around the bar, but that's all they do -- and hope that they'll make it all the way around. But to securely make it around, especially if you're going to do another trick immediately out of it (like a cast or squat-on), you need to flip your wrists so that your hands are on top of the bar. Doing this also forces your shoulders over the bar so that you don't end up falling backwards from the way you came.
Like most tricks in gymnastics, the front hip requires most gymnasts to learn how the correct way feels. You'll probably have to keep trying repeatedly before it becomes consistent. Don't give up!