I keep a spare phone in my drawer ready to replace my main device if needed. Is it OK to just keep it in my drawer for months, or should I take steps to take care of it in order to keep it in good working condition?
Keep only the necessary apps that you use on the spare phone and update them once in a while.
Turn on the spare phone once in a while. I once let a (spare) phone sit without turning it on and didn't charge it and it bricked. (It was rooted so I was able to revive it by restoring a nandroid backup)
I am presently using my spare phone as a house phone by using T-Mobile's Digits app.
Use it as a spare media player. I use mine around the house. Watching netflix or listening to Spotify. Why wear out the battery on the primary phone when you have a spare! LOL
I picked up a pay-as-you-go sim for my 'beater phone' just to have mobile data. I never use the number. I do have to recharge it every three months because the plans expire, but $10 every three months is worth it for the convenience.