Some of this has been covered before in related threads. But I think it is worth repeating on it's own thread.

Atlanta Hobbies covers it very well in their hint note below. This is important if you want to have your batteries last over 100 cycles.

In a crude analogy, think of your batteries like your muscles. Flex your biceps (you know, just like you do in front of the mirror to show yourself that you are still a "manly man" even if you are past 50  ) .... now hold that flex for a few weeks or months. My guess is that you will quickly gain some empathy for the plight of your fully charged batteries sitting around in the "flex" position all winter long. 

From Atlanta hobbies
 
Winter Battery storage tip!
If you need to store your batteries for the winter or are not going to be using them for a while, (Rudy note: >7 to 10 days) make sure they are fresh and flight ready when you need them. Li-Poly batteries do not handle being left in the fully charged or drained condition very well. To insure you have good batteries for spring please follow these basic
steps:
1. Charge the battery for storage to 3.8v to 3.9v per cell.
2. Check the voltage with a Digital Volt Meter once a
month while they are in storage.
3. Re-charge them as necessary to keep them at 3.8v to
3.9v per cell.
4. Keep them stored in a cool, dry area with a stable
temperature of 40 to 70 degrees F.
I hope this will make it easy for you to jump right out
and resume flying when spring comes.

PS: Atlanta Hobbies is a great source for electric stuff. I have been to their warehouse and they are a very knowledgeable and friendly group of people to do business with. During lunch breaks they just step out their back door and FLY, nice work if you can get it.    ..... Link:

       http://www.atlantahobby.com/store/pc/home.asp

Regards,