Sorry I'm late to the party...
You actually have a few good options. I had the same issue, but part of it was that my OEM battery was three years old. The dealership said my aftermarket stereo was to blame, but I seriously doubt that. They said the stereo was also why all of my door lock actuators eventually failed, which is clearly stupid.
1. You can get a deep cycle battery, such as the Optima Gel (ideally larger than is suggested for the vehicle).
2. You can find out what is draining the battery, but this is often an exercise in futility.
3. You can add a second battery (either serial or parallel), but it is not likely to fit in the engine compartment.
4. You can keep the car on a trickle charger or battery tender.
I went with an Optima Yellow Top one size up from the standard for a Crosstrek... love it! In retrospect, I might have gone with an even larger battery (such as an Odyssey), but I'm not disappointed with the Gel.