Charge the phone to 50%ish, then turn it off and store. When you want to use it, just top it off, and before storing it away let the battery drop to 50%, then store it back,
From what I know, keeping the battery at ~50% means the electrodes in battery will be in sort of eqilibrium, hence will cause minimal wear. Just make sure its not in a humid place.
I assume by back you mean not always in use aka ON so here's what I follow for my backup phones.
Charge them 100%, keep them working aka ON for 8-10hrs after charging (subject to battery condition else it might drain down to zero in that period) then switch them off completely.
2-3 months down power-on and charge again. This is how I'm able to keep my backup phones healthy till date.
I now have kept inserted backup sim cards as well in them so when I want to recharge etc. I use the phone for a good one day and follow the methods back before they are good to sleep back in cupboards!