General NiMh storage rules apply I think. For long term storage, 40% charge is recommended and cycle the pack every few months. (I couldn't tell you how to get 40% though)
1) Don't run pack down too low on a regular basis, when light starts to dim noticeably is a good time to recharge. Don't keep running it until there's no light at all.
2) Don't leave in this discharged state for more than a month
I should add;
3) For maximum performance, store in discharged state and cycle every month.
4) Storing at 100% charge for long periods of time will reduce performance but won't do any permanent damage. (A few cycles will bring back max performance)