Batteries are best stored at temperatures of 20°C / 68°F.
More specifically, a battery provides more power at higher temperatures due to faster chemical reactions, which also speeds up self-discharge and reduces the battery's life.
At low temperatures, the opposite happens, and the battery can actually hold its charge for longer. However, since chemical reactions are slower, the battery also produces less power. This can lead to the powered device to run out of power much faster, until ambient temperatures rise. At low temperatures batteries also risk freezing to death.
So the first step to prolong battery life is to avoid storing them in places that can get very cold, such as basements or cellars during winter, or places that can get very hot, such as near a window during sunny summer days.
If you're willing to delete all of your savegames, you might as well store the cartridge in a cold environment (beware of frost damage), though that may not necessarily be good advice depending on the battery. The cartridge itself won't mind getting boiled or frozen (but it won't like getting roasted), since SNES cartridges are made of Nintendium.