Opposite Foot Kick
Kickers and punters often have back problems. This is because we spend so much time swinging only one leg. This strengthens some muscles in our legs, hips, and lower back and leaves the same muscle groups on our opposite side weak. This leads to back problems that can ruin a season. Once when I was seeing a chiropractor for my bad back, he recommended that I spend some time kicking with my left foot (right foot if you are left footed) to strengthen the other side of my body. Ever since I have always kicked some balls left footed to help avoid back problems.
Another thing you can do when you are kicking alone in the off-season is kick your normal field goals and then when you go to retrieve your balls you can punt them back with your opposite foot. It is obviously hard in the beginning, but you would be surprised how fast you improve.