In the past year I've done a lot of experimentation with different forms of exercise, all to different ends. Overall I'm a better shooter because of it, but improving my shooting was never the primary goal - more to make myself more well rounded as a warrior.
Cardio - did the eliptical for 4 weeks, 40 minutes. After the first month, I moved to the treadmill, eventually going from a 16-minute two mile to a sub-14 minute two mile. Did this for about 3 months, and lost 20 pounds in the process. Advantage to shooting: Tire less quickly during cardio-centric activity. This could be running to the fight, moving to cover, or movement supported by fire.
P90X - Began this at the beginning of the year, and wrapped it up in March. Far and away the most rewarding overall fitness program I've ever done. Lots of variety, and enjoyable enough that it never got boring, which is a major key to a fitness program for me. Didn't lose weight on this, which is okay since the cardio I was doing bottomed me out. Program takes 6 days per week for 60-90 minutes if followed properly. The program varies, but generally its 3 days per week weight lifting, two cardio, one yoga. For shooting, the increase in overall fitness allowed to me to hold both rifles and handguns in a ready position for a longer period of time with no fatigue. This minimizes the sideways "8" wobble and creates a steadier overall base. If I had to guess, I would say the yoga (fine muscle control) and weights (stronger core/base) made the most difference.
Crossfit - Lately I've been continuing some parts of the P90X and introducing new elements from crossfit into my workouts, mostly in the form of sprints, circuits, and exercises that are new to me. No noticeable effect on shooting yet (haven't fired a weapon in a few weeks), but my upper back and upper body strength has noticeably increased. This means its easier to walk for longer periods of time with a plate carrier and combat load on board, which in turn helps shooting in context. I do like the practical focus of crossfit - many of the exercises are geared at the idea of overcoming obstacles with the basic guideline being that if you can reach it, you can climb on top of it. Still new to the whole thing, but will look at joining a gym specifically for this when I get home.