The biggist problem I've had is storing my compasses too close. It will not take long before they are no longer accurate. I still see the following storage packs, and wander if they are too close:

http://www.silva.se/sv/Produkter/Compasses/Utbildningspaket/?productId=%7b3D859035-CF48-4A53-BCF7-FB3B29AEB4EF%7d