My camera uses a rechargeable lithium ion battery and an SD memory card. When I'm out shooting and at home, I normally keep an extra of each in a zip lock type of plastic storage bag (the card is first put into the plastic holder that it came with). Is this a safe way to store them? If not, is there a better way to avoid any problems?
SD cards are very safe when stored in their original plastic container at room temperature in a dry place. The same is generally true of Lithium Ion batteries. Protecting the metal contacts on the SD card and battery will prevent the majority of damage to either device, either through corrosion, physical damage, or electrical short. Storing the memory card (in its plastic case) and battery in separate plastic bags should protect from most problems.
Optimally the battery should be stored at about 60 degrees F and roughly 50 percent charge. The exact charge can be difficult to determine, and the charge percentage needed increases with temperature, but in practical use this is really only a concern for long-term storage.
Personalĺy, I would never store or carry ANY camera cards and ANY batteries together. Card info storage operates electronically and batteries can have a strong (albeit local) magnetic field. To put one's faith in the card, and any files saved on it, being unaffected by being in close proximity to batteries isn't something I'd recommend.