I learned this lesson early as one of my units stored this way came out working but was rusted EVERYWHERE inside and was eventually it's demise. I learned the biggest problem was the daily temperature differential.
I have had great results with two different methods;
1. A night light (7watt) placed inside can dry and warm the air just enough (inside) to keep the humidity down.
2. If no power available, or for more strength, you can use a commercial product known as Dri-z-air, it is silica beads that are placed in a plastic device that attracts the moisture. Cheap and available at Wal Mart.