1. Loosen the tension on the strings. Detuning the strings a step or so will protect the guitar from any gradual damage caused by constant tension being placed on the neck while the guitar isn't being played or handled. However, you shouldn't completely detune the guitar or remove the strings--guitars are designed to have tension on the neck from strings, so completely depriving your instrument of that tension is a bad idea. Just detune the guitar slightly.
2. Heat is your enemy. You should never keep a guitar, acoustic or electric, in any environment that might get extremely hot. Heat will damage the wood of the guitar, melt glue, damage electronics, and just generally do a bang-up job of ruining your axe. You don't want the guitar to get too cold, either--cold can do even more damage to the wood of the guitar. Don't store your guitar in any attics or unfinished basements, as these are the types of environments that encompass both extremes and may have fluctuating temperatures. If your guitar is stored in a slightly cold place, that's fine, but let the guitar sit in its case in a warm room for a few hours before playing it again.
3. Watch moisture, too. Excessive moisture or dryness can also destroy a guitar. Store your guitar in a non-humid place that also isn't excessively dry. A closet on the ground level of a house is a good place. To protect against dryness damage, you may want to polish your axe and apply a little bit of lemon oil to the wood of the guitar, body and neck, before storing the guitar. Store the guitar in a hard-shell case, and make sure it's not in a place where it's going to get bumped around a lot.