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This is always a hot topic. My take is always do you really want to have to pull out every card and use it every three months just to keep it open? I absolutely do not. I also don't want to close accounts because you never know when they are going to come out with a new card and maybe you want to PC to that. In my case Citi DC, Discover It, BoA 123, Cap One QS and Chase Freedom all started out as other cards. Once a year is the sweet spot for me. I pull them all out at the start of each new year and use em all that month. I pay them all off right away and throw em back in the SD. If anybody wants to close my account due to 12 months of inactivity I don't want their card anyways and obviously I don't need it if I'm not even using it. Credit cards should be making your life easier not creating more work.
I think someone mentioned earlier to buy $0.50 e-gift cards from Amazon. That's probably honestly your best bet to show use on your Visa/MasterCard/AMEX/Discover card. As there is always something on Amazon you may need eventually. I've got about 40 cards myself and basically I just put $20.00 once a month on my Amazon balance. It's a pain in the ass but if you've got time to do it once a month it's not too bad.
I have a lot of store cards from places I don't really shop but those can show use by buying a shirt on sale every 6 months or less. If you sign up on the store sites you'll usually get % off coupons or $ off coupons with free shipping. I've went up to 6 months on a few cards with no closures/credit limit increases resulting from no use. I got a CareCredit card that I've never used for 13 months; not sure why they haven't closed that yet but I may use it this month for an eye exam. Haven't had one in awhile.
Good luck! I would say using a credit card that never gets used at least once a month is obsessive, every 6 months is pushing it and every 3 months is just about right. Obviously you use your normal use cards many times a month.