In order to get rid of back pain when skiing you have to start working your back muscles as well as your hips and core muscles.
Weak hip muscles or muscular imbalances around the hips will transfer stress to the lower back, so this is an important part of the solution because of the forces that act when skiing.
The same goes for the core muscles whose job it is to stabililze around the spine. If your core muscles are untrained and inactive, they cannot do their job when skiing, and your back will be more at risk of an injury.
You can also work the lower back by doing variations of hyperextensions, supermans and bridges.
One final important thing to do to avoid back pain is to make sure your hamstrings and hip flexors don't get too tight, by doing some proper strecthing exercises:
Hamstring stretching example

Lie down on your back and grab your one leg at calf level. Pull it towards your chest in an extended position till you feel the stretch on the back side of the leg.
Hip flexor strecthing example

Take a big step forward, both feet pointing forward. From here bend the knee of the front leg while you keep the back one extended. Keep your chest up as your move your hips forward and feel the stretch in your hip flexors.
Read about the most common skiing injuries by clicking on any of the subjects in the submenu above
You can reduce the risk of skiing injuries by preparing for skiing with a skiing specific program and consistent training . You may want to check out this dvd with ski specific training programs to do at home or in the gym.